Well, he says he offered to pay.

Was this before or after he asked them to fit him so he could buy his bike somewhere else? My guess is after. You have to bear in mind that once you've insulted somebody like that by asking for a fit so you can go buy your bike somewhere else, offering them money afterwards is likely to just feel like adding insult to injury.

Maybe try this next time?

"Do you offer fitting services?"
"Yes./No./Only as part of the sale of a bike or frame."
"How much?/Thanks anyway!/I was planning on buying Product X for $Y elsewhere, what could you offer me?"