Since there is so much doubt and uncertainty here, I suggest a sit-down conference between buyer, seller, and agents. Your position is how do we close this deal: satisfaction of the tax bill, adjustment to the sale price, buyer credits, and so on.

Maybe there is seller remorse? That's not enough to get out of a deal. If there are tax problems, seller needs to provide documentation.

The goal of your conference is to reach an agreement to close, or cancel the deal.