The Peacekeepers might not be the ones offering the group their freedom. Possibly a single person in their ranks that has found the old information on the group and thinks that there is more hidden away in a deep dark dungeon some where. Sending the PCs into the dungeon is likely to be be the only way to get the tomes.
A merchant might have paid one of the lesser peacekeepers a few coin to offer the PCs a way out. However if they take it the Peacekeepers will be hunting them for a while. Which the merchant can easily move his merchandise though town while the 'guards' are busy hunting the PC group.
The Peacekeeper leader found information that a rare artifact from the old times is deep in a local mine. Something that could cause a LOT of damage. He is not willing to send his own people in for it. The prisoners can go in for it, if you offer then a chance for freedom to retrieve it.
The Peacekeeper leader knows nothing of the offer, the second in command wants his boss out of the way to go a different way with the peacekeepers. Move from the shadows into the light and make the Peacekeepers the local town guard. Sending the PCs out will cause the leader to stumble in his belief that what he is doing is correct. This group of adventurers past broke out.