Will not at first agree but will grovel and pretend submission. Will write note but encoded to warn. If no other opportunity has arisen, the traitor will say he will take note. Whatever number to test your PCs. If they do, they will not be allowed to leave alive since they are to be blamed with it.
Dead men tell no tales.
The negotiation delay will give some time for setting up defences if it occurs to players to hedge. Too bad if they don't. If the PCs can hold out 2 days, a concerned father will arrive with relieving force. I made an earlier post on the net frp conference on moral dilemmas and here is the detail. In reward for services, a sage offers this little test to a group of PCs.