I'm glad you haven't spent it all yet....just in case.
I've never heard of a case where SSA 'forgot' to pay the lawyer out of your backpay BEFORE issuing your check.
I would wait for communication from SSA to see how they want to handle it. I hate to think there could be a lawyer out there trying to double dip with an unsuspecting disabled person, but....better safe than sorry. If SSA says to mail them a check, or that they will take a portion of your monthly checks going forward until the debt is repaid, that's one thing, but I don't think you should be conned into sending the lawyer a check.
Good luck with getting this straightened out.