Sadly, in the end it will be the tech that did the works that gets screwed the most. It may not be your fault, but it's not the techs fault either. They do the work they are assigned and have nothing to do with it being covered under warranty. I've worked at dealerships and have been screwed like that myself. He'll have his hours/commission pulled for that job if neither you or the warranty pay out. If it were me I'd either come to some agreement with discounts or ask for the car to be returned to it's original condition and fix it yourself for cheap.
Say WHAT????????????????????
Here's an alternative - call Buick and file a complaint about the dealer.
The OP has no obligation, legally or ethically, to pay a dime. Once done, it is unethical for the dealer to ask for payment after the fact.
From my other posts, you know that I often side with the dealers, but in this care they are just plain wrong.
Will the tech get screwed? Probably,but even if you paid now he would still stay screwed. You think the service manager will call the tech in, say "Hey, the mark came in and paid" and hand the tech $50.