Anger is a natural, instinctive part of your physiological survival mechanisms.

Fight/Flight/Freeze = Anger/Fear/Disassociation

Anger is meant to give you the courage, determination and strength to defend yourself or those whom you love.

Whenever you are feeling anger, and it is not the result of a mental illness (yours is clearly not, you have clear reason to be angry), it means that your brain has identified a threat, and you need to deal with it. Your anger is there to help you deal with it, give you motivation and 'oomph' to deal with it.

I get the impression that you don't want to give him permission, you don't want to give him the signature - all of which makes perfect sense, all things considered.

However you also seem to have fear and shame issues. You are vulnerable because this was your father and he emotionally wounded you. You are vulnerable to being emotionally manipulated into doing something you don't want to do.

In comes anger - a loud and clear signal that you are being threatened. Anger makes its appearance to let you know that this is not healthy, this person is a threat, and you shouldn't do anything you don't feel comfortable with.