Assalaamu alaykum wa rahmatullaahi wa barakaatuhu. Dear Scholars, how are you? May Allaah reward you for your good deeds. My question today is the following: Several years ago (about six or seven), when I was a university student, somebody sent me some money through the bank. The sent amount was 200 Ethiopian birr (about 10 US dollars), but when I went to the bank, (The bank actually is the one which works with interest.) the cashier gave me 300 Ethiopian birr (about 15 US dollars). I took it because I suspected it might be what was sent for me. Later on, however, I became sure that I had taken extra money. Moreover, I do not know that cashier in person in order to pay it back. I think it is not proper to give it to the bank itself because the burden was on that cashier (he may have paid it for the bank). So dear Scholar, what shall I do now? May Allaah reward you.

All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger. 

If you are sure or you predominantly think that you took more than your right because of an error from the cashier, then you should go back to the bank to inform them about what had happened. If you find the cashier, then you inform him and give him back the extra money, but if you do not find him and the bank informs you that the cashier is liable for any shortage, then look for the cashier, and if you do not find him, then give that amount in charity while intending the reward for him, as a way of acting upon what you are able to do, as Allaah says (what means): {So fear Allaah as much as you are able.} [Quran 64:16]

Allaah knows best.