I think I understand what you have asked. Please tell me if I am wrong.
Back in 2008 he was in arrears. So the government attached his tax return and eventually received the money. Also in play was an SSDI award, that the government eventually attached for arrearages.
So, your ex is upset because he may or maynot have had too much money taken out. He would like that back, and has come to you for it.
This is the child support agencies problem, not yours. They will adjust his account and return to him extra money that is over and above what he owed. LET THEM HANDLE IT !
If you get involved there will not be a paper trial, and it may still be fixed, but this time at your expense. If the agency decides you were paid too much, they will take it out of future months, or ask for it back from you. If you pay him now, they may ask for it back too, and then where will you be.
Again, LET THEM HANDLE IT !!!!!!
You can contact them if you like, but you must really keep them in the loop, and let them handle the money.