Jamie did not screw Brienne out of a deal.
The deal was, B gets The Blackfish to surrender, and J will allow her and the Tully army to go north. B failed, so no deal.
Edmure then was convinced to surrender via the threat by J to kill E's wife and child, and EVERY remaining Tully. Every ****ing one.
When E surrendered, the command was given to all to "Drop your weapons"
There is no chance that B (Or Pod, for that matter.) were going to disarm, thus they needed to sneak out, thus the boat.
J now has the "Tully Army" as his prisoners, and I saw zero indication that he was letting them go north.
PS. The only marching Tullys I saw were weaponless.