@metincan3606 usually the proper reason would be that you are actually unable to do anything more. Sure he might be able to kill one or two or even all of them, but what are the odds that they would not be able to wound you in return? Or target your companion? Or flee and ensure that they can damage you where you can't protect (friends/family/social standing)? Or simply flee and claim you lost a fight recorded near the line, so she owe them her body (I couldn't see a Line where they verified the contests result, and afaik monsters wouldnt show up if it was there either).
The whole chapter felt stupid imo, noone would actually ever agree to such a bet (not to mention author seems to have forgotten she is pretending to be male? Probably easier for a male to earn the money by dungeon delving than with their body, unless their culture is secretly very different than ours and author forgot to tell us). And this is true for both sides, it is obvious noone would ever bet all their money like that, so it is just as obvious they intended to shirk the deal if they lost.