I'm a bit late to the conversation, but glad OP got it worked out.
IN TEXAS, because I'm only licensed in Texas and this is where OP is posting from, here are my thoughts: There were good ideas and some less well-considered responses.
If this comes up and the buyer/seller can't work it out between them, talk to a real estate lawyer. If the money warrants action, send a demand letter. When a demand letter from a firm goes out, it changes the game from checkers to chess. Seller might reconsider their position.
Next up - file suit for specific performance. IN TEXAS, lis pendens requires a live lawsuit before filing in the real property records. IN TEXAS, suits based on contract, the prevailing party may recover attorney's fees.
Relief in a suit - IN TEXAS, specific performance is frowned upon (read, unlikely) if monetary damages will make the prevailing party whole. Sale of real property is one of the few cases where it is likely to be awarded. But even here, if the buyer and seller are dealers ("flippers"), then money damages could be awarded.
As for timing, show up to closing with money in escrow. If/when the seller doesn't, have the case ready to file the following morning. Seller should actually breach the contract before filing suit. If the underlying suit has merit, counterclaims for slander of title are unlikely to stick. Doesn't mean that seller won't bring them, just that they have a hard time proving it.