First one needs to determine from whom the note likely fell. The gemara[1] discusses the case of one who finds a lost item in a store, distinguishing between the part of the store that is frequented by customers and the proprietor’s area. If you found the credit note in the customer area, you can assume that it fell from a customer (unless you found it in an area near where the worker’s operate), especially considering that credits are usually ripped up after they are redeemed or kept in the register to check the day’s proceeds.
Next, we must discuss whether you should try to return the note to the person who lost it. This depends on whether it has a siman (a distinguishing characteristic, so that one can prove that it was he who lost it). If the amount of the credit note corresponds to the value of a specific item that was purchased and returned (as opposed to a coupon that is like a gift certificate with a set denomination), then this constitutes a siman.[2] In that case, you should put up a sign in an appropriate place in or around the store, or you should give a customer service worker your phone number in case someone comes to look for the note. If the store is unhelpful or it is clear from the type of store it is that you will not be able to return the note to its owner, you can assume that he will give up hope of finding it. (It would have been nice of you to wait a few moments to see whether someone was looking around the store for it, although this was probably not halachically required.)
Is it permitted and/or moral to use a credit note that the store wrote for a different person? First, it is important to note that the store’s guidelines on the use of the note, as well as any pertinent local laws, could make the following analysis moot. If there are no such guidelines or relevant laws, it is important to determine the status of a credit note.
A credit note is, in many ways, like a partially open check of the store (i.e., the recipient is not specified). This type of “document” was prevalent in previous centuries, and the poskim called it a mamrani. It was usually written by a borrower, who gave it to a lender to ease collecting the loan, as he could collect directly from the borrower or sell it to someone else. There is a lengthy discussing in the Pitchei Teshuva[3] regarding the opinions of the Acharonim about a case in which a lender was given a mamrani[4] and lost it, and he then asked the borrower, who admitted he had not yet paid, to pay him without receiving the mamrani in return. One of the main issues was whether the lender could write a shovar (receipt) that effectively dictated that whoever would present the mamrani to the borrower would not be able to receive payment for it. This would save the honest borrower from potentially paying twice. The Tzemach Tzedek is cited as acknowledging a custom that, in such cases, an announcement would be made in the local shul/community that anyone who possessed this mamrani of the borrower in question must produce it within a certain amount of time. After that, it would no longer be valid. The poskim’s general orientation is that a mamrani is not like cash or an object of value, but is rather a device to facilitate the efficient payment of loans, either to the lender or to the person who bought the mamrani from him. Accordingly, it would be wrong for a finder of a mamrani to seek payment as if he were the lender or one to whom the lender transferred rights.
The perspective is likely much the same for credits at local or small stores, where relationships have been established between the proprietor and at least many customers. Consequently, if a customer said he lost the note, the proprietor is likely to believe him and act accordingly. It follows that the note is not like money, which if lost is lost, but is rather a reminder of a debt. In view of this, one who uses someone else’s credit note is cheating the store; the proprietor likely honored the credit already in the absence of the note.
In contrast, if one loses the note in a large, impersonal store, he will not receive the credit, and the store will have “gained” by not paying its debt to the customer. Another person who redeems it simply replaces the deserving recipient. If the finder cannot locate the recipient and redeems the note, the recipient does not lose more than he already has lost. In that case, the store is also not losing, because they should be paying anyway; it should not make a difference to whom they pay as long as they will not have to pay twice. The store views their note as something of value that the customer can use or give as a present or sell. Thus, if the finder cannot return the note to the one who lost it and the store will not give him a refund without it, the finder may keep it and use it like one who finds an ordinary object that has no simanim.