Perhaps there is some loop hole in the law that gives you the right to change the mural since you technically still own copyright. The change of course would be the removal of your signature. I suggest that you read up on the copyright laws
http://www.copyright.gov/ . Then you can go to the manager and instead of asking for permission, tell him that as copyright owner you find it necessary to make some adjustments to the painting and then tell him what day and time you will be there. If you talk with confidence and throw a couple of legal copyright terms at him I bet your bluff will work. Then all you have to do is show up and take care of the problem.
Good Luck,
Mary