Answer:
The buyer can sue for specific performance of the contract. In real estate, in order for the buyer to be able to sue for specific performance, he/she must have all the money (or mortgage) ready to finish the transaction. It is very difficult for someone to sue for damages for not wanting to complete the sale of a house because houses are unique in a way that similar houses in different neighborhoods or even streets might be worth a lot more or less.
When you sue for specific performance, the non-breaching party will request that the other party performs their side of the contract.