You pay income tax on the gain and that gain is calculated as Net proceeds to you minus your Adjusted basis.
If what you're saying here is that you want to have a "stated" selling price of $300K but will only receive $250K because of a "concession", e.g., an agreement that you'll contribute $50K in order to fix the foundation, then that $50K can be view as an addition to your basis or a reduction of your proceeds. Either way your profit is the same.
