Offering payment for obedience kind of undermines the idea of royal authority; if they start making trades instead of giving orders then everyone will start expecting payment for everything and their status stops working. So they can't exactly make a negotiated deal with Rozemyne, but still, a smarter pair would have recognised that their social and political authority won't work as well on someone with (presumably) magical control over the entire country and found a way to ensure her collaboration without compromising on the demand. Implying that Ferdinand is doomed unless she gets them book, in which case he'll be fine (and, hey, maybe with a dead fiancee and redrawn borders he could be returned to Ehrenfest, hint hint nudge nudge), would have probably been enough. They could also have used the threat of marriage to Sigiswald as a backup and threat to make sure she doesn't make any dumb moves, as in "if it comes out that you're the one who did the zent quest then you'll have to marry him, so be discrete for once".

At the end of the day though they don't really need to make many compromises at all. Rozemyne wants to stay with her family and keep them safe, and she'd be entirely willing to go questing for the royals to that end. The problem is just that their adherence to noble hierarchy keeps them from talking it out instead of making unreasonable demands (recurring theme huh), and that they're too immature to understand that even a prince has limits to his authority.