Sold a canadian car to a us LA resident and he ask me to lower the price on the bill of sale
Can I report the fault on prove he did wrong

How did he do wrong? He asked you lower the price on the bill of sale, you didn’t have to. That was a choice you made.

Did he ask you to convert Canadian $ to US dollars? As of today, a Canadian $ is only worth .74 US $. So, a car listed at $10,000 Canadian is worth 7,400 US $

He really did not do the "wrong" if you lowered the price for him, then you were the one who did wrong by providing a false document.

So unless you want to be investigated, I would not worry about him. On this