2002 Ford Escort Trans fluid leak after engine rebuilt

I had the engine rebuilt on the car the other week to hopefully get another year out of it (it has run pretty good for over 6 years).

A few days after, I noticed that I had been leaking transmission fluid in the drive way for days. I took it to them to look at this morning, and the guy said that a valve needed replaced and I think he said they needed to drop the axle (or soemthing similar). I asked "are you going to charge me for it?" He said it would be a couple hundred bucks.

I never had a fluid leak until after they gave me the car back. I got a warranty from them for the engine job - can I demand that they do the job for free? Or is this typical of all mechanics, honest or not? After spending a couple grand, this couple hundred extra seems a bit much.