Thanks for the replies!

I have tried to compromise and offered to return the keys in exchage for payment but he won't budge.

He also stated in his last email that the "fatal error" that came up on the computer occurred while we where in the building because we pulled out the power cord. We did not pull any power cords out so it's so frustrating to be accused of it! I was also reading "fatal errors" can happen for lots of reasons.