With regard to damages: they *could* sue you for quite a lot, but they probably won't because of the prohibitive cost to them. If they bought your house for $500K, and equivalent properties have gone up to $600K in the meantime, they could sue you for the $100K difference that they've had to pay due to your breach. This would, however, be quite difficult and expensive to prove to a Court's satisfaction.
TheAnalyst said:
what specific performance they can claim? damages? That is what I want to know a bit.
"Specific performance" isn't damages, it means they can ask the Court to enforce the contract as written, ie Get the Court to transfer ownership of the property to them, on the terms contained in the contract, despite your objection to fulfilling your end of the contract. Yes, this is possible, but quite rare - it usually gets to the point where the vendor sees they're going to lose and proceeds with the transfer, because if it got to a point where the Court enforces specific performance, you'd probably lose the property
and have to pay the buyer's legal costs, and possibly any damages relating to the delay.