Motivated Seller, How to Avoid Long-Term Gains Tax?

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I have a scenario for a potential purchase. Basically the guy bought his house a longggg time ago for $20k and wants to sell, and we agree on a $450k sales price. Is there a way to pay him $270k "on paper" so he doesn't get hit with a long term capital gains tax at 15% and then pay him the remaining balance of $180k over time through a consulting agreement or other instrument where he would avoid the tax. On top of that, could I set my basis at $450k when I sell it? I wouldn't want to do him a favor to avoid tax, but then have my basis be $270k and then I get hit but a huge tax bill when I sell for say $550k, which would wipe out my profits. Is this above board or is there another way to approach this?