This is the old negotiating trick of putting one more "minor" item on the table after the table has been put away. The usual reaction of the victim is something like, "Oh, what the heck, I've got to get going. Sure go ahead."
Wittingly or not, your manager wore you down and then threw in a zinger, fully realizing that you lacked the energy, desire, or time to open the discussions again. Score one for his team.
If you agreed to his last minute change in the benefits, then you would be acting questionably if you were to now go back on your word and try to reopen the negotiations. The best message for you in this scenario is to be on guard for this type of negotiating tactic in the future, and to be extra vigilant when dealing with your manager.
If he tries this eleventh hour tactic again, even if you are at your car door with key in hand, put your key back in your pocket and go back to the negotiating table.
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