Suggestion:
1) Process for de-allocation to be used. System checks so that there will not be unnecessary allocation. Allocation will only happen through unconfirmed transactions done by the users.
2) User can manually deallocate the allocated stock which is against order/customer using Manual Stock Deallocation screen. For manual stock deallocation goto SCM-Sales->Order execution->Manual Stock Deallocation screen.
3) If need to deallocate any old allocation then add a despatch and drop the line, system will automatically deallocate the same.