Credit card debt

Amex turned my outstanding debt over to a debt collection agency (of course my account was already closed) and I sent (March 5, 2008) my payment to them in the form of a money gram express payment for the full amount of $1,810.30. On the money gram express payment form I put all the necessary info and also put Account Paid in Full. The debt collection agency received the payment and told me they could not accept it because of the note placed on there, so I call Amex and they told me that the debt collection should have no problem accepting payment. Amex advised me to have money gram request for the payment to be returned to them (which would have taken 10 business days) and once I receive it to send the payment directly to them and even provided me an address to send it to. I made the request of money gram and the process was set in motion. On May 20, 2008 I called money gram to see if they had gotten my money back and was told not as of yet. I then called Amex to inform them it may be a couple more days and was told that they had already received my payment of $1,810.30 and that my account had been credited. I asked for a letter reflecting that. I never got the letter and I called money gram back to stop the process of getting money back. I receive two statements one (3/28/08) showing that they charged me twice in the amt. of $1,810.30=3,620.60 with Agency Remittance on it but amt owed and the new balance showed $0.00. The statement showed that they credited my account the amount owed to them and that I had a $1,794.96 credit. The second (4/17/08) statement showed that my balance and the amount owed was $0.00 and they credit balance refund in the amt. of $1,794.96. I put the money in my account thinking they had not taken the payment yet. Now it's is July and Amex Collections dept. is calling saying they never received there money. I call money gram and they call the debt collectors and they rec'd the payment on 3/5/08 and payment was accepted and put to my acct. on 3/7/08. Now Amex is saying that on 5/29/08 there was an NSF on 5/29/08 and they reopened my acct. with them. Correct me if I am wrong since everything they sent me has a zero balance and my money gram paperwork has paid in full I am right in thinking that I do not have to pay since I already paid and due to a clerical error on that was made by Amex it not my fault they did not get there money since they are the ones who sent me a check back and still credited my account?