seller asks for cash deal with contract

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I am trying to buy a self-storage building. The asking price was $179,900, I worked it down to $163,500. The seller wants to set the contract sale price for $150,000 and wants $13,500 cash on the side. This will help him with his capital gains. He wants $5,000 up front, and $8,500 at closing. Are there any risks (legal or otherwise) with this deal?