The dragon IS the man to whom the King negotiated his daughter's marriage, and whether the dragon tricked them about his species or not, the marriage is necessary for peace. The evil vizier (who wants the treaty to fail as part of his bid to depose the King and usurp the throne for himself) and a foolhardy but mostly goodhearted noble youth (who is crushing on the princess, and may have been her boyfriend before her arranged marriage) plead with the party to rescue her from the dragon, and leave out the "arranged marriage" part of it.

Whether the princess is against the arranged marriage and cooperates in the rescue, is resigned to the arranged marriage because she knew it would be her duty no matter what (and at least her husband turns into a hot guy for consumation), or has actually fallen for her husband (who has all the best gifts and actually works to make her happy), the party has some tricky choices. Actually, the first would be the trickiest, because "princess doesn't want to be married to her arranged husband, but the political rammifications of breaking the treaty are myriad and bad" is a decent plotline in and of itself. Still best if the dragon in question is not cruel to her, but nevertheless requires the marriage for the treaty.