Actually, he doesn't have his deposit slip, he would have a receipt of the deposit. The bank would keep the deposit slip, and when researching his request would realize the error. If you have withdrawn the funds, then the bank would come to you. If your account was closed, depending on the bank, they would either try to contact you, or pursue via the local authorities. There are numerous court cases with this error, and in each and every one the bank wins. Imagine if this was you, and the missing money was yours. Wouldn't you want the other person to do the right thing?
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WC done for US - let's hope we fare better in the Tour de France