You need to see where the fee is coming from. if it's added to your purchase, then it's the store charging you, and it's out of your control. If it's added to your bank statement, then it's your bank and you need a new one.

This is a totally unrelated thing to the buy and return. ATM fees for the merchant are not based on the dollar amount, they pay the same thirty five or fifty cents (whatever their agreement is) if you use your card for a pack of gum, or you use it for a pack of gum and $200 in cash. You actually help them by reducing their cash they have to process.

Buying and returning something with a miles earning credit card forces the merchant to pay a service fee twice, which of course they will pass along to the rest of us.

As others have said, it's a moot point as they usually won't give you cash back for a credit card purchase, if they do it's usually against thhe terms they have with their merchant account, as most include lines like the following (from one I have)

You will not, under any circumstances, pay any card refund or adjustment to a cardholder in cash.