I did this once, long time a go, with AMHR, on one of the first horses we bought. They allowed it--the work order had the seller's name & address & membership number and I wrote the check to send with it. No problems there at that time.
With AMHA the same thing did not work--said the non-member fee applied. If I'd given the money to the seller & she sent it in it would have been fine, but because the name on the check didn't match the name on the membership, it didn't work for them. Their reasoning was they couldn't let "just anybody" use their membership, or they'd get no money, everyone would use one person to do all the registry work..... honestly, that was the most foolish reasoning I'd ever heard....it wasn't "just anybody" it was one of the parties directly involved in the transaction, which isn't quite the same as if A were using his membership to pay for a transfer from B to C.... In view of that I would have to guess that what you're wanting to do in this case will not be allowed. If you want to use your membership for the transfer with AMHA you need to have your buyer give you the transfer fee and then you send it all in.