How would you handle this monetary gift?

[rcsmom]

So for Christmas my amazing mother-in-law got us new convertible car seats for the twins which meant that I had two basically brand new extra infant seats. We are not planning on having any more so after checking with my sister who gave us the seats I offered them up for free on FB. I didn't have any takers for a week or two even after tagging some people I thought might be interested.
After a week or two I got a message from my (distant) cousin. She married a man who attends our church along with his whole family. She said that some of his cousins were having babies this year and they would like to pass the car seats along to them. Perfect! So I gave them to her about two weeks ago.
Now the issue. The other day I got a card in the mail from my cousin's in-laws. We do know them- they usually sit right behind us in church and are very nice people. It was a thank you card for the car seats and they included $100 bill. I do not know if my cousin knows that her in-laws sent us this. I really don't want to keep the money as the car seats were a free gift. I cannot for the life of me however figure out how to send it back tactfully without offending anyone. Any ideas?