I was going to ask "does this need a marketplace?" and link to a list someone was maintaining a few month ago of folks who offered to match donations made to the ACLU, but it looks as though someone turned that into a little web app, anyway:
http://matchingdonations.us/. That said, the harder part of this is probably getting people to make that offer to begin with. If this becomes a common thing, I'm sure someone will build a platform around it. If someone told me they'd donate to a charity of my choice in exchange for having dinner or coffee, I'd probably jump at the chance and match their donation, as well.