Borrowing or depositing without witnesses

A loan is either witnessed (and in some opinions even one witness) or a bill.
In any case, I am not aware of any such prohibition on deposits.
Some differences between the cases can be suggested. For example, a borrower is usually in need and therefore there is more concern. Perhaps even in a loan, a distinction should be made between money and objects (money is intended for spending and objects are less useful. Objects are identifiable by the owner and money is not).
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Asks:
Is it forbidden to lend without witnesses/a bill, even for loans of small amounts such as five or ten shekels?
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Rabbi:
In principle, yes, since the Sages did not divide in their ruling. Although I must say that it is not customary to be strict about this, and usually the lender simply forgives the borrower if he forgets and does not repay (and thus does not fail him even if he does not repay). If you do not forgive in any way, it is better not to do so.
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Asks:
 
Is a cash deposit similar to a loan in the sense that it also needs to be given in a promissory note or witnessed? For example, a deposit in a money market account or a deposit in a bank. 
 
Additionally, is there a prohibition on keeping a deposit slip from the Money Laundering Authority at home after they returned part of the deposit to me? ("A person is prohibited from keeping a loose slip in his house," Ketubot 19:1)
And what would be the answer to the previous question if the deposit deed is signed only by the deposit recipient (not signed by two witnesses), does this invalidate its validity?
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Rabbi:
I don't recall any reference to a deposit in the poskim, although there seems to be a similarity. But a deposit cannot be withdrawn, so there is less chance of forgetting. The question is whether the original regulation is concerned with intentionality or forgetfulness (see Shach and Sama'a, beginning of the 6th century, which discussed the issue of a TA, in which there is no concern that it will be withdrawn intentionally, but there is a concern that it will be forgotten). If the deposit is withdrawable (meaning you don't have to return the money you received, but only the same amount), it is a loan and not a deposit.
But there is another difference between a deposit and a loan, in that the borrower is usually in need, while with a deposit, he is actually doing the depositor a favor. Perhaps someone who is not in need has no fear that the deposit will be denied. This is true even if the deposit can be spent.
Bottom line, I haven't heard of anyone enforcing this (even with easy loans these days, although the jurists are puzzled about it), but it's still best to deposit with witnesses or other evidence. There's no problem with the bank because there's evidence on their computer and in their forms.
 
A bill signed only by the person making the payment is a bill for all intents and purposes (in the words of the Gemara and the Poskim: handwritten).
The prohibition on keeping a bill of exchange in one's home raises the same question that I raised above: Is it a fear that he will forget or that he will sue again on purpose? I think that when a prohibition is imposed on the person himself, then it is more likely that it is a fear that he will forget (after all, if he does it on purpose, then he will also pass by and stay in his home). Therefore, in order not to forget, it is written on the bill that part has been repaid, or you give the tax collector a receipt for the amount that has been repaid.