But, all else aside, if a customer tells you three times that you gave them too much money in change, and appears to take the situation very seriously, it would be best to try to either understand the mistake or unwind the transaction. To folks of a certain cast of mind (I don't claim it's a good one or fortunate to have), a troubling transaction is much more costly than a few days of groceries.
I can understand the contrary view, and why it might be held, but in shopkeeping, if one side invokes the "make this transaction right" rule, particularly when it's not to their advantage, that does need to become the prime objective of the encounter.