"continuously dripping wet " so he has either a leak or has wet vac'd the seats and there still wet, i would myself tell him to bring it back leave it with you for a few days so you can dry it out and check for leaks, run a hose pipe over the obvious and you have done all you can do, i bet a mars bar he has washed the seats and there still wet.
if he does turn into an ass then at least you have shown goodwill, a few days without his car i would go 50/50 he wont even bother bringing it back.
one point i would ask is how do you know he has insured the car in the trade ?