Don’t do it.
He just wants to pay little or no state sales tax. If he wants the car, tell him the the Bill of Sale will be accurate or you’ll find someone else.
By documenting the car at a lower value than the actual sales price, you are committing tax fraud.

Depending on the state, if the value is way way out of line they can charge the buyer fair market value. So yeah, $10k for a $100k car, they’ll flag that in an instant.
But if you wrote up $90k for a $100k car, they probably would not.