I’ve had this happen before.
The law is on your side. You have a valid, enforceable contract, so the seller ‘must’ sell.
That said, buyers don’t have to buy.
Meantime, you record a memorandum of agreement, or notice of agreement, or notice of sale, or whatever works in your state/county, and cloud the title. This does not require the seller’s signature. It just requires a signed purchase and sale agreement, which is NOT recorded.
The seller will not be able to sell to anyone else, without first clearing your lien.
The seller is gonna get all cramped up here when he discovers that you know better…and discovers the recorded notice. [insert evil-sounding laugh here]
Meantime, you’re in permanent control over whom this property is sold to, because of your signed purchase and sale agreement. [insert another evil-sounding laugh here]
Maybe you could get the seller to buy you out of your contract …and “pay you not to buy.” [insert gasping, evil-sounding, belly laugh here]
Hope that gives you some encouragement.