You're underthinking if you would even consider it not subject to SE tax.

Contractual relationship between manufacturer and car-dealership employee:  None. 

Contractual relationship between manufacturer and guy who buys his products:  Picture the truck delivering the goods, the dealer writing the check.  How much closer do you need?

If you buy widgets from me for $100 each, but sales are slow for me this month so I give you $10 for each one you sell before the 31st, what do you call that?  It's a rebate.  It reduces my cost of goods sold.  

Maybe you are leaving out some essential facts.  Is the business Widgets Inc, and Larry the owner pays himself on a W-2 and maybe a K-1?  That's a close encounter of the second kind -- but he's still diverting business income (or reductions in business expenses) to himself.