The answer depends on whether you are looking at the legal issue or the practicable one. Legally, you could probably sue and force the seller to sell if he tries to back out after the contract for sale has been signed and ROFR completed.
The practicable answer is don't bother doing that. If the seller backs out, you get any deposit back and can just go and buy another contract. The cost of actually pursuing a legal claim, including hiring a lawyer, would easily make that contract far more expensive than the amount you ever wanted to pay.