A fixed compensation! The prices are still fluctuating in the markets of Dis, and even demanding thrice the original payment would hardly suffice to protect the asset.
But that, he can hardly say to her; some other excuse must be found to reject the term.
"The mortal is not here, party to this contract though you seemingly intend him to be. Are you hiding things from him, I wonder? And if so, I wonder, do you wish him to see a compact plainly mentioning three Wishes and a hundred pounds of spellsilver?"
He can't just suggest a term about market prices though, or a price reflecting lost gains to the devil who must sell; that will make it too obvious what's really going on.