What Say You?

I saw this post on FaceBook today. A business friend of mine is looking for advice, feedback and opinions.

Need your help: My son was invited to attend a friend's birthday celebration at an amusement park (only 3 boys were invited.) The dad sent me a text saying it would cost me $77 to cover my son (with dinner, souvenir and a present we're looking at at least $100)

I sent the dad a text saying I can't afford that amount for my son to attend a birthday party. So he sent me this back: "I'll cover him. Maybe he can work it off around my place or something? Washing car, cleaning with the kids, etc?"

I'm not sure how to respond to him. Does my son need to work for another parent to be invited to a birthday party? Isn't a birthday party an invitation to have fun?
I'm confused on what to do. Any help would be appreciated. Thanks :-)

What say you?