I agree with what the others said- capital gains on your profit on the sale, and what you do with the money is your business. But mutual gifts (he gifts you the payment of property taxes, you gift him a discount on sale price) are taxable the same as outright sales, with the added feature that you'll attract the attention of several taxing agencies if you try to put through a sale with an artificially low price. Part of this comes from your friendly Department of Revenue, which collects a transaction tax on sales- they are very active in auditing anything suspicious, and have some hefty fines and penalties if they catch you. Plus, they notify IRS of any monkey business they find, and the nightmare continues.
Its not worth it. Do an arms-length transaction, pay the capital gains, and move on. Confine your creative transaction skills to stuff where there is no paper trail.