Solid deal
The rewards which these snakes offered were way overdone. The oldest men offered their finest daughters, for example, all of which were young women. Although that sounded appealing, Gustav later figured out that these snakes never mated while in their human-like form!
In any normal occasion this wouldn't freak him out, but it didn't seem appealing to man-handle snakes through sex, especially since they were eight-to-fourteen feet long! Mammalian people wouldn't dream of that, and the snakes wouldn't dream about mating with humans, either.
For that matter, regardless of how pretty the women were around here, he had to reject every offer. This in turn, got the whole tribe very frustrated and angry!
They insisted on being even and fair, regardless of the cause. Perhaps it burdened them that they held on to a favor which they owed to the terranidans, for generations! They did not want to accept another burden, and that explained why they insisted so greatly.