The Flaw:
When the motel owner gives the money back to the original party...he in effect has not 'retired' the prostitutes debt because he is still "out" for cost/profit for the room to the prostitute...no sale was made to the "new customer" as he didn't like the conditions of the rooms, so he took his money back, and the Motel owner did not "keep" (bank or use) the hundred so no debt was erased.. The basic flaw here is he's still a hundred short...I'm not much of a math or accounting guy, but I think this is correct...Tyrannosauruss