Completely different opinion: You called for a roll and it succeeded. The amount offered is the amount accepted. There's no reason to walk back a success to something you framed and that the PCs succeeded at. I'm quite sure that if they had failed you wouldn't be here asking how to keep the mercenary available to the PCs.
I think the weretiger agreed to be paid but not to "how much", is that right? If so, I think minimally they would want their expenses covered for a month, even if the party didn't need them that long, so the minimum that would need to be offered for them to even consider working for the party is 60 gp. Considering that the weretiger is an expert lookout (with keen hearing and smell, no less), I think they'd want more than that. Double or even triple that amount could give a bonus to any check that you might ask for to resolve whether they accept the offer. If the weretiger has enough class levels or anything special that pushes their CR up into the 5 to 10 range, then that minimum amount of inducement should be more like 1,100 gp.