I'm sorry to tell you but you have to drain and fill yet again. Of course, you could just refuse and sell the car to me
Just reuse the gasket and fluid and don't be a crybaby
(JOKING, PLAYING AROUND, JUST KIDDING, ETC).
Quote from Sexton's writeup:
'The good news is the repair can be made while the transmission is in the car in the case of a 126 it's pretty easy - in other chassis the trans needs to be dropped just a little although people have reported success, with some effort, of doing it to 123 and 107 chassis with the tranny in place. It's not a terribly expensive repair either, the part is about $100 and it'll take you a couple of hours. While there's a few steps to this it's really only a little worse than changing a spark plug and you don't need of be a master mechanic to do this.'
Don't be afraid. If you could, please post photos for the forum