AITA for refusing to give back a parking ticket I found in my friend’s car?

I found a violation notice while borrowing a friend’s car and withheld it until she repaid fuel money; late fees accrued.

I defended the action as the quickest way to enforce a real boundary. It solved the immediate problem, but it also imposed a consequence the other person had not agreed was mine to choose. I am left deciding whether being justified about the grievance made my method proportionate.