He's going to bad mouth you to everyone anyhow, no matter what you'd have done, no matter whether there was a fire or not, if he'd have gotten it back the next day, its how these types operate.
I'd take a portion of the bet saying your not going to get a dime, and still end up being the ahole.
If I hadn't received payment, I'd send him the bill for the full amount, then take him to court, let the judge hash out what your owed, you have phone records of the harassment he gave you, show pictures of your burned down shop, if insurance is involved, you'll have that paperwork to show your waiting for them to act, if no insurance was involved, then ask the judge what you owe the guy. or what's fair? You have a receipt for the pump purchase, doctors instructions, a buddy to explain in court what took place, how things went down and if nothing else, let the judge decide what your owed, then there shouldn't be any moral dilemma on your part, what the law decides is how it goes down, end on decision.
I'm not thinking it would too hard to prove you went above and beyond what anyone on earth would have done, how you were treated, and how things went down. I don't have a lot of faith in the legal system, but right now, I'm thinking your being had by a complete jerkwad, but that's just my opinion.