With every car I buy/sell that doesn't have an accurate price, I write 4 bill of sales:
(2) with the correct amount that I paid/sold (one for me, one for the buyer/seller)
(2) with the agreed amount on the title (one for me, one for the buyer/seller)
Disclaimer: I have flipped one car in 5 years, so I don't do this anymore. All of this stuff is a risk and honestly if I flipped a car now, I'd do everything the correct way. Not worth the risk.