Of course you can say no. It still means Bob is an asshole. Bob, unlike you, has many more options. He deliberately chooses to take advantage of your lack of options, even though behaving like a normal, decent human being and helping you for free by simply giving you the goddamn extinguisher would entail no great cost to him.
People like Bob are not out there to help others. They're out there to win.
You've just described the business model of a Roman character called Crassus.
He invented the fire brigade.
He also invented a fire brigade protection racket: if your house was burning he'd offer to buy it at a knock-down once-in-a-lifetime price before putting the fire out.
It was a solution of sorts. But maybe not the best of all possible solutions.
People like Bob are not out there to help others. They're out there to win.