IMHO, there are two ways to handle this problem. The dealer charged you a fair price to perform a relatively simple job. They only completed part of the job, so they either owe you a full refund, or they should do the job correctly.

FWIW, the simple maintenance jobs (oil changes, transmission service, etc.) usually go to the least experienced techs'. The whole thing may have been an honest mistake. It's possible the guy who worked on your car didn't even know the TC had a drain plug.