Is boron used in fire extinguishers and windows?
5 shots dry fired is not enough to ruin a decent gun. RTM on the oil, its probably normal.
First make sure he did the job and that the new keys work with no problems. I'll assume all is well there. Send him a check for $40 with a copy of your ad that says $40 in it. If I remember right most lock kits come with a new strike plate included so he shouldn't have needed to buy another. Write something along the lines that this was the ad you responded to-to change the locks for $40. If he checked it out and didn't want to do the job all he had to do was walk away. If he saw that it was going to run more than $40 he should have contacted you for approval. That's what all other repair men do when digging into repairs and he's no exception. He did it so here's the $40. If he wants to sue for the other $50 he says he owes, go ahead and let him. You and I both know it wouldn't be worth all the time to file a small claims case over that little amount of money anyway. Heck, just to file would cost him most of that $50 anyway-LOL.
I wouldn't open up that gun unless you already have a full working knowledge of the workings of the gun. The dry firing can hurt the air rifle for sure, but you will likely still get years of good use out of the gun. Keep shooting it, but don't dry fire it any more.
If it shoots fine, don't worry about it.