Max On Prepayment Penality Seller Can Charge?

What is the maximum a seller could charge for a prepayment penality for commercial property seller financing? I am aware of 6 months on residental property. Would there be a requirement difference between residental and commercial. I have yet found anything related to commercial property on seller financing. When I recommended to the seller 6 months ... he said that was "not enough money". The seller wants to charge 1 year prepayment penality. Interest payments would be around $3,500 per month x 12=$42,000 would be 1 year prepayment penalty if loan paid off within 5 years. Cheryl Lopez