What should have happened is that they make the exchange AFTER they get all the oil they need. Just don't "hire" the guy until everything is fully ready and you already have a stockpile of oil. Then you give the rights to the oil to him.
@ShenMegami The thing is, there's no true romance with mermaids. They jump ship as soon as they encounter another male with a higher level. That was mentioned in the novel I believe when he said the line "perverts will be perverts" a few chapters back.
Since this is going to me mentioned waaayyy later (at least in the novel), I thought I'd mention it now. Lanuvel purposely leads the hero party into trap infested corridors, just to see what will happen. He has his own method of telling whether that's the case or not that he uses later on.