Oil leak thought it was rear main seal

Had a oil leak I figured it was rear main seal took to mechanic so he could inspect I called he said oil pan was leaking, I said repair that and replace rear main seal too. I paid him for both oil pan and rear main seal pick my truck up now it leaks transmission fluid. If he took the transmission out to replace rear main seal would have he not have noticed transmission fluid in the bell housing and that it needed a new seal? I do not want to have stinking thinking, but I am thinking he thought he solved oil leak problem with the oil pan and never put in a new rear main seal would have paid more for new seals for the transmission since he had it a part already. I see that they do not cost a lot. That is my dilemma. Now he wants another $400.00 to put seals in transmission. Thank you so much.