I have my 1994 Chevy Blazer up for sale

I have my 1994 Chevy Blazer up for sale. I also needed to move the vehicle to a location immediately as the time ran out on it for the location it was originally at. An couple who are interested in purchasing the vehicle offered to buy it and have it moved to their house while the contract for payments were going to be drawn up. I did give them the keys ( I have worked with the wife for about 7 years and trust her). In the process of looking it over, they decided not to buy the blazer as it had too many issues for them at the time.
But my problem is it needed a new battery (even though I had driven it over to their house) as they said it wouldn't keep the charge. They purchased the battery without my knowledge or permission. So when they said they were not interested, they dropped the bomb on me about them purchasing the battery.
When I emailed them to ask for my keys back so I can move the vehicle to a different location, they want their $110 that they spent on the battery.
How do I handle this situation?
They did this out of their own idea, without permission from me first.
I do value the friendship, but I feel I have been placed in a very awkward position.
How do I resolve this?