How did you manage to get a posting if you had signed off?
Just because you apply for early release, doesn't mean you will get it, I would not bet on your final day being the end of September.
Your old Unit will probably not want anything to do with you, and your CoC at your new Unit will be more than a little bit pissed off to have to deal with you getting out.
As to your new Unit authorising leave, they might find a reason to not allow it, although that is unlikely. If both Units are on leave, how do you propose to put in for leave or apply for early release?
It looks as if you have painted yourself into a shitty corner. Hope you can get yourself out of it OK.
If you have signed off your leave and course entitlement change if you apply for early release depending on how quickly you want to go after signing off, i.e. you may not be eligible for all the GRT/annual leave and courses you have applied for.