Thank you very much to the Rabbi for the quick response.
If nothing is specified other than the prices, for example:
• Price for 30 days: 100
• Price for 60 days: 101
Would there be an interest problem in that case?

Usually, among professionals in the industry, things are handled in a shortened and concise way, because transactions are done on a daily basis.
With us, they ask what the price is, and they always try to argue it down; usually they lower the price. Sometimes the seller stands by his price and says that this is the final price for that merchandise.
Still, a basic question comes up: when there is no clear market price, where exactly is the halakhic line?
Seemingly, one could argue regarding many items that they do not have a "market price."
For example:
• A professional, such as an electrician, offers pricing that varies from one service provider to another and also changes based on negotiation with the customer.
• Likewise in cases like buying computers for an office, the supplier offers a high price that gets adjusted through negotiation, or different suppliers have different prices.
In such cases too, can the price be treated as a "market price"?

There are opinions that say yes and opinions that say no. Since this is a rabbinic prohibition (because this is a sale, not a loan), there is room to be lenient.
Nowadays almost nothing has a market price. That applies to the examples you gave too.

Thank you for answering my previous questions.
While reading the answers, a few more questions came up for me on the subject of interest:
1. Selling to overseas customers – delayed payment to the supplier:
We export goods to customers abroad, and sometimes customers request a certain product. In such cases, I arrange with my supplier to give us the product here in Israel, and we send it to our customer abroad, with the understanding that the customer may return the product within 30 days. If the customer does not return it, I pay the supplier within 60 days from the date of delivery.
Since in this case it is something like a product loan for the purpose of sale, is it permitted to offer the supplier a reduced payment if I pay him within 45 days?
2. Payment in dollars versus shekels – concern for interest:
In a case where a supplier gives me a price in dollars, but we agree that payment will be made in shekels in 60 days, according to the representative exchange rate on the day the invoice is issued – is there any concern of a prohibition of interest in that?
3. Library fines – is that interest?
The municipal library has started charging fines for returning books late. Does paying such fines constitute a prohibition of interest?
4. Regarding market price – if there is some good rule of thumb for what counts as a product with a market price.
I could also claim that toilet paper has no market price, since at Shufersal Sheli it costs X whereas at Rami Levy it costs Y, etc.

1. I didn’t understand why you say this is a loan. You aren’t returning the product itself to him but its price, no? The fact that you are offering a reduced price proves that this is a sale and not a loan. You are basically giving him a guarantee on the sale. Even if you were returning the actual product itself, in my opinion this would still be a sale with responsibility, not a loan.
2. There are different opinions on that, but in my opinion there is no problem here. It is basically payment in dollars, except that you transfer the dollars to him through their shekel value. This can be compared to a loan of a se'ah for a se'ah, which is a rabbinic prohibition that has various leniencies. All the more so here, where these are sale proceeds.
3. What does that have to do with interest? It is a penalty for late returners. Also, under the Libraries Law, you are not paying a reader’s fee. It is only a fine payment and not interest on anything else.
4. I think that today almost no product has a market price, unless there is an actual government law on the matter.
Here I saw someone who wants to argue that one can sue based on the price of a basket of goods: https://www.toraland.org.il/%D7%9E%D7%90%D7%9E%D7%A8%D7%99%D7%9D/%D7%9B%D7%9C%D7%9B%D7%9C%D7%94-%D7%95%D7%9E%D7%A9%D7%A4%D7%98/%D7%9B%D7%9C%D7%9B%D7%9C%D7%94-%D7%95%D7%94%D7%9C%D7%9B%D7%94/%D7%90%D7%95%D7%A0%D7%90%D7%94-%D7%91%D7%99%D7%9E%D7%99%D7%A0%D7%95-%D7%9E%D7%91%D7%95%D7%A1%D7%A1%D7%AA-%D7%9E%D7%95%D7%A6%D7%A8-%D7%90%D7%95-%D7%A1%D7%9C-%D7%A7%D7%A0%D7%99%D7%95%D7%AA/
I disagree with him, because even a basket has no market price.
There are claims that even if there is a range of prices in the market, it is forbidden to sell above the highest price, but that claim also seems problematic to me. First, there is no market price here, and in such a case there is no overcharging. Second, according to that, the one who set the highest price always violated the prohibition – so why should I, who come after him, take him into account?!

1. We write the document in the following style:
Document: Confirmation of transaction terms
Date: 1/12/2024
Product price: $500
Terms:
We are entitled to return the product within 30 days from the date of purchase. If the product is not returned on time, we undertake to pay its price within 60 days from the date of delivery.
I receive the product on loan, with the option of selling it to my customer.
Even then, does the Rabbi understand that it is fine to negotiate with the seller and say that I am willing to pay $450 on day 45 during the negotiations?
2. Price changing according to the exchange rate:
I agree that the price depends on a rate that is not known in advance, neither to the buyer nor to the seller. Since the rate changes and is not fixed in advance, there is no interest problem in this case.
3. Fines for late return of an item:
A similar principle can be applied to other items. For example, if I lend my printer to my neighbor, I can have him sign a contract obligating him to return the printer within 48 hours. If he does not meet that condition, he will incur a fine of 5 NIS for each day of delay.
According to the Rabbi’s words, this is not interest, since the fine is for the damage caused by the delay and is not related to the value of the object itself.
Early-payment discount:
However, I found a similar question on the Kav Halakha website:

https://kavhalacha.co.il/%D7%90%D7%A7%D7%98%D7%95%D7%90%D7%9C%D7%99%D7%94-%D7%91%D7%94%D7%9C%D7%9B%D7%94/%D7%93%D7%99%D7%A0%D7%99-%D7%9E%D7%9E%D7%95%D7%A0%D7%95%D7%AA/%D7%94%D7%90%D7%9D-%D7%99%D7%A9-%D7%90%D7%99%D7%A1%D7%95%D7%A8-%D7%A8%D7%99%D7%91%D7%99%D7%AA-%D7%91%D7%AA%D7%A9%D7%9C%D7%95%D7%9D-%D7%9E%D7%A8%D7%90%D7%A9-%D7%A2%D7%9D-%D7%94%D7%A0%D7%97%D7%94-%D7%94%D7%A8%D7%91-%D7%9E%D7%A9%D7%94-%D7%99%D7%95%D7%A1%D7%A3

The site claims that it is forbidden to request a discount for catering services when the raw materials are not in the seller’s possession at the time the order is placed.
However, according to what the Rabbi said, in a case where there is no fixed price for catering and the parties are allowed to bargain over the price, one could argue that there is no prohibition of interest here.
Did I understand correctly?
(I am Ashkenazi.)

1. I understand that “I receive the product on loan” is not the wording of the document but your interpretation of what is written there. But that itself is what doesn’t seem right to me. You receive it for agency in selling, not as a loan. The negotiation between you is over the premium due to you for your agency. And if that is the wording in the document itself, then change it.
3. I didn’t understand what the connection is between that question and our discussion. A fine for returning a library book late is not interest in any sense. What is the principal that this would be its interest? Where is the loan or the sale here? In the catering case there is principal.

Hello Honorable Rabbi,
I wanted to think about what the Rabbi wrote for another day and see if it became clearer.
I come from the jewelry and diamonds field and I deal with shipping goods abroad.
The method I described is actually called memo.
I asked ChatGPT
to define for me what a memo is, and I got the following answer:
“Receiving goods on memo is a common commercial method in the diamond and jewelry industry, in which goods are delivered to a customer (usually a wholesaler, retailer, or business partner) without an immediate purchase, but on the basis of an understanding that the goods will be returned if they are not sold or are not found suitable.
Principles of memo:
1. Ownership remains with the supplier: the goods are not sold but loaned, so the supplier remains the owner until sale or return.
2. Purpose of examination: the customer receives the goods in order to show them to end customers, check suitability, or inspect quality.
3. Limited time period: the memo process is defined for a set period, at the end of which the customer is required to return the goods or pay for them if he sold them.
4. Agreed in advance: a memo agreement usually includes a written document detailing the terms of the loan, the value of the goods, and the conditions for return or payment.

In light of what I wrote, if I negotiate in advance about the price and the payment dates, and I receive the goods on memo, is there really no concern for interest here?
I’m deliberately sharpening the point because it’s very important to me to understand how to conduct myself correctly from a business perspective.

As for what I wrote about the catering from the following link:
https://kavhalacha.co.il/%D7%90%D7%A7%D7%98%D7%95%D7%90%D7%9C%D7%99%D7%94-%D7%91%D7%94%D7%9C%D7%9B%D7%94/%D7%93%D7%99%D7%A0%D7%99-%D7%9E%D7%9E%D7%95%D7%A0%D7%95%D7%AA/%D7%94%D7%90%D7%9D-%D7%99%D7%A9-%D7%90%D7%99%D7%A1%D7%95%D7%A8-%D7%A8%D7%99%D7%91%D7%99%D7%AA-%D7%91%D7%AA%D7%A9%D7%9C%D7%95%D7%9D-%D7%9E%D7%A8%D7%90%D7%A9-%D7%A2%D7%9D-%D7%94%D7%A0%D7%97%D7%94-%D7%94%D7%A8%D7%91-%D7%9E%D7%A9%D7%94-%D7%99%D7%95%D7%A1%D7%A3
The purpose of that was only to show that the rabbi there thinks that payment for catering – which seemingly has no market price because it can vary – still raises a concern of interest,
whereas if I understood the Rabbi correctly, today almost nothing has a market price except only a price-controlled product.

What you described says explicitly what I wrote: the goods are not owned by you but by the original seller. He defines it as loaned to you, although I am not sure that is the right definition. You are his agent to sell the item. Either way, this is not a loan.
As for the catering, it does not say there that there is a market price. There is the caterer’s own price, and he gives a discount relative to his own price for earlier payment. And when you have goods of that type, even if the quantity is small, there is no prohibition here (in the catering case he discusses when you have goods but the consumer does not want them right now because they perish, and even about that there is a dispute of opinions).

First of all,
thank you to the Rabbi for the response.
I’m glad to hear that this is not a loan, and therefore I am not violating the prohibition of interest even if I used leverage to get a better price because of earlier payment.
I had a feeling it wasn’t, but I wanted to be on the safe side.
Is it possible to make something like a heter iska for my entire business so that I have halakhic protection that I’m not violating anything?

The point I was trying to make through the catering post is that it has no market price.
From what I learned from the Rabbi’s previous post, almost everything other than a government price-controlled product is something without a market price.
So why did that rabbi there (on the site dealing with catering) get into the whole consideration of whether he has such goods or doesn’t have them? He should simply answer that this is a price with no market price, and therefore there is no issue of interest here.
What did I miss?

Regarding the examples of the library and the printer – can one say that every fine is not interest?
For example, if I’m late paying the electric company, I have interest charges.
(I assume the big companies have a heter iska, but for the sake of the question I’m asking.)

All right, I’ve lost the thread in this discussion. It’s hard for me to keep track of it in parallel with the other discussions here. I’ll summarize briefly:

The question of market price is discussed mainly in relation to the laws of overcharging. There it is quite clear that nowadays almost no product has a market price. Regarding interest, the matter is subtler, because if there is your own price, even if it is not a market price, when you charge a higher price for delayed payment or a lower one for earlier payment you may run into interest problems.
I no longer remember clearly what the original problem here was. It is permitted to set a different price for later or earlier payment if you have no clear price of your own and there is no market price. If you do have a clear price and you change it, that may be prohibited.
As for the transaction you described here, I am not sure a heter iska is relevant here. You are not talking about a loan but about agency for sale. That is precisely the definition of iska, since the money or goods belong to the sender and are given into your hand, half as a loan and half as a deposit, and then the profit of the giver is on the deposit half. But in your case you are not doing a business venture for the sender; rather, you are sent to sell for him a specific item at a specific price. The discussion is not about how you will transfer profit to him, but how much you will pay him if you return the goods.