Your obligation as a taxpayer is to correctly report your taxable income. If you do that then you're pretty much bullet-proof with the IRS.
In real estate transactions the word "concession" typically refers to the seller agreeing to pay something on behalf of the buyer, like fixing the foundation as I mentioned, paying the property tax the buyer would otherwise owe, and so forth. So, mechanically, I'm not sure what form this concession will take in your contract. Maybe we're really talking about a "gift of equity" here because as you're selling to a relative or old friend.
But it's hard to comprehend how a house with a Fair Market Value asking price of $250K can support a mortgage of $300K.
