Sell a product for $7 but only allow "recharge" in $5 increments. It pretty much guarantees that you will have tons of accounts with random dollar amounts laying around in them, never to be used. Even for regular gift cards, the counter party would be beholden to honoring them at their full value, as they should.

Besides, nothing it stopping you from rolling monthly treasuries and using those as a bank. Something people with enough money to exploit this proposed "gift card bank" would almost surely already be aware of.