Okay, and according to which rate does he have to repay the loan? The rate on date X or Y?

The whole idea is that you return what you borrowed. There is no rate. You borrowed one Bitcoin, return one.

Do you hold that with a dollar loan there is no problem either (contrary to the accepted view), or that Bitcoin is better than the dollar (because it is traded “everywhere”)?

In my opinion, even with dollars there is no interest, and it is not like se’ah-for-se’ah. True, the halakhic decisors disagree about this, and some wrote that the dollar is like merchandise measured in the local currency. But Bitcoin is not the currency of any particular place, so it seems that its law is like the shekel, even according to the views that do not say this about the dollar.

By the way, a good piece of advice for someone who is stringent about a dollar loan: keep one dollar on hand, and then there is no prohibition of interest in a dollar loan, just like the rule of se’ah-for-se’ah.

If “Bitcoin is not the currency of a particular place,” then obviously there is no permission to lend Bitcoin for Bitcoin.

What is obvious to the master is difficult for me. I understand that in your view, the world is not a place. Strange.

I quoted your words, if you didn’t notice.

As for the substance of the matter: if someone damages Bitcoin (if such a thing exists; for example, he caused his fellow to lose the code, and this requires further analysis under the laws of indirect causation), what does he have to pay—money or Bitcoin?
If he has to pay money, then lending Bitcoin for Bitcoin is prohibited.

As far as I know, aside from sporadic cases, people do not buy goods in exchange for Bitcoin, so it is hard to say that it has liquidity/sharpness. If international trade were to move to Bitcoin—as today it is based on the dollar—then one could talk about Bitcoin as a currency, but right now it is hard to say that it is a currency any more than the dollars that the Lubavitcher Rebbe distributed were currency. No one can seriously claim that the coinage of the Lubavitcher Rebbe circulated throughout the world and therefore had the status of a sovereign’s coinage, and by the same token it is hard to say that Bitcoin’s coinage circulates throughout the world and therefore has the status of currency.
Recently a book by Dr. Dror Goldberg was published about money, “Gold, Paper, and Crypto,” which discusses these issues. He argues, based on his research on the English colonies in North America, that what makes money into money is the ability to pay taxes with it. It is not only taxes, but also the legal backing that the state gives to contracts in its currency. In the State of Israel you cannot pay taxes with Bitcoin, and in my opinion there is no backing for contracts paid in Bitcoin. And since even in the international sphere it is not really a currency, we are left with the definition of Bitcoin as merchandise.

Yaakov,
You quoted yourself, not me. The first words in your comment were a quotation, and I was referring to the last ones.

Y.D.,
In my lectures on the chapter “HaZahav,” I explained that liquidity/sharpness is not a criterion for currency. Sharpness comes up only in the Talmudic passage that seeks to determine what is the currency and what is the merchandise in a transaction of currency against currency (gold versus silver and the like). There is no discussion at all there of defining the concept of money in itself. I seem to recall that I also wrote about this here on the site.