Putty will not work, probably. Or it won't work for long. You can't really seal the cut end of two pipes together with puddy. There is lots of heat and plenty of vibration in that area.

Most likely you will destroy the nut, and the ability to seal the pipe to the manifold anyway, in the process of removing it.

Trust me, it is not worth the effort. If it has not come off with an open end wrench, then it is not going to go quietly into the night.

Just get it out of there and move on with the repair.

Michael