agree with above, and will add that if it is a large amount and the customer easily should have recognized the error deposited to his account, you may wish to consider ending the relationship with them.
I had a deposit made to my account in error once, low 5-figure amount, clearly not mine (i was in college and rarely had $5, much less 5 figure balances). I notified the bank and they said they would take care of it. After a week it was still there, so i moved it to a savings account to accrue interest (back in the day when interest rates were over 10%, yeah, i'm old). Took them almost 3 months, and they were quite indignant when they realized their error and basically accused me of fraud. I simply went in and had a nice discussion with the irate manager - showing him my letter and their response when i notified them several months back. His tone changed greatly, when he saw the response had come from his boss.
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