It is our bank's policy that you can cash it, but the customer must bring their account current with the funds. Any extra funds, the customer my walk out the door with, just as you might do with any other check. The fact that the check is drawn on your institution only provides you with the knowledge that the check is good to go. No worries on returns (well, for the most part). That customer is responsible to keep their account current with your banking policies.
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"If there's nothing wrong with me, there must be something wrong with the universe..." - Dr. Beverly Crusher