I guess at this point its not negotiable, you have dropped your load and your leaving
Mistakes can happen. I've been paid shredder for a load of heavy melt, signed the slip and had cash in my hand before I looked at it, and then told the lady at the window the problem, and had the problem taken care of. If the yard man has seen it, all it should take is for him to verify what it was. That is, if they value your business....