If this happened to me here where I live, I wouldn't sue....and I wouldn't huff and puff. And call me immature, but I absolutely would not renegotiate a signed sales agreement just because they made a greedy mistake. All I would do is record my memorandum of contract, tell everyone that looks that there is a signed sales contract out there......then disappear off of the face of the earth for a while and make them come find me.
At that time they will either close on the property....or pay me 5 grand to go away. I could live with either scenario. All the while being very elusive and taking days to respond back for anything.......
You can always give in at the absolute very last moment if you HAVE to own this thing. But to me at this time this a game of screwing with someone until they cry uncle or you lose interest in the whole thing. Either way what do you care? You'll have another great deal to look at real shortly with people that won't dick you around.
Ron Legrand had a great saying that has always stuck with me........"If you take you time and set things up so that you can't lose......its hard to lose.
Real Estate isn't always pretty and nice, big boy pants are needed once in a while. Not caring gets easier the more successful you become.