Given that the purchase of their new property is contingent on the sale of this, the seller has his balls in a vice. He's going to have to sign or else his house of cards is going to fall. I saw this happen once with a neighbor that was over-extended on his real-estate deals. He damn-near lost the whole kaboodle and this would have undoubtedly triggered a divorce as well. $325K in hand is a powerful incentive.
Henry Seale assumed you had a title search showing a clear title.
I will ask that question.
Do you?
With no contingencies like timber?
How about another move to consider:
Find the plot on the river he is looking at and work a deal to at least avoid your potential tax consequences...