if he owes you then you have every right to collect that debt. deduct what that amount is from what he sends you, then remind him of the previous deal and tell him you are even at that point and if he wishes to purchase the current item from you then he has a balance due. give him a reasonable amount of time to follow thru and if he does not then return his remaining money via a method that requires a signature by him and him only (I would deduct any fees required for his refund also). I would photo copy everything, money order or check from him and any refund going back to him for proof of the transaction. keep everything in a file and wait for his next move.