Do you know for a fact, as in there is a signed lease with a tenant who has given deposit, that he has already rented the apartment for July?
Security deposits can be used for money owed on a lease agreement in some states. It is not always strictly limited to only physical damages to the property. You would need to look up your state's laws to find out what your state says concerning the deposit.
Either way, you need to decide once and for all what you want to do and then do whatever it is you are going to do. Either leave to your other home and put this behind you or continue to drag it out for the last few days/weeks and complicate the issues more and more and more by doing things on purpose now just to be a hard ass and create a pissing contest and have all this follow you to your new home. What purpose would it serve? What purpose does it serve to keep calling the LL and trying to come up with different deals once one has already been agreed upon? Have the company pay the $600 or don't. Tell the landlord that you are staying through July or hand over the keys at the end of June and be done with it. Either way pick a direction and do it and deal with whatever outcomes stem from it.
Going back and forth on this board trying to convince other people who disagree with your view that you are right is pointless. You will only agree and listen to the posters that write what you already have set in your mind is correct anyway.