The best is to talk to the professor about being not authorized to work but if he insists, work from the date he wanted but with no paperwork and get the payment only after Aug 25. For the hours worked before Aug 25, talk to prof to raise the per hour rate for first couple of weeks after aug 25 to match those hours and thereafter use the normal rate. People worked on SSN during F1 since they have no choice. As long as you have choice, it's better to be in safe.