Some history to set the stage for the question, if I may...

An elderly couple lives behind us (who once owned all of the land that our subdivision is built on) and we have had a good relationship over the 7+ years we have lived here. Recently they got a fright from a robbery attempt and needed to move some of their outside lighting to feel safer. Their daughter is arranging it. They needed to get our permission for a utility easement on the property, as well as permission to trim one of our pecan trees (that this couple actually planted many years ago). This was a "no-brainer" thing for us, since it is a safety issue.

Now the etiquette question/issue...

After they got all the permissions straightened out, the daughter came over and gave my wife a card. My wife said it wasn't necessary, but she insisted that we take it because she "wanted to do this for us." After saying thanks and parting ways, my wife opened it and we suddenly felt quite awkward. Inside was two $100 bills.

Would it be insulting to give the money back?
If we are keeping the money, should we write our own thank you note in return?

I've never really faced this sort of situation before, so any real etiquette guidance would be appreciated.