Apartment manager didn't assess my apt until 2 weeks after move out

[Chris Clemptor]

When I moved in to my apartment, I decided that instead of giving a refundable damage deposit of 1000 I took the option of paying a non refundable 300 fee prior to move in so I wouldn't have to deep clean the apt when I left. 

I moved out of my apartment 2 weeks before the end of the lease. ( paid the rent in full) I had told 2 managers in person, 4 days prior.  I spoke to them both at the same time verbally that this was my intent and they said "ok".

When I moved out I contacted maintenance to let them know I finished moving and they could now access the apartment for a turnover, and they said "ok"

Two days after moving out I called the office again from out of state and let them know I had moved out and where my keys were located they said "ok."

10 days later I called the office again in order to find out the name of the renters insurance policy they use so I could cancel it.  It was at this time the manager mentioned they hadn't been to the unit yet and was scheduled 2 weeks after I moved out.

A month later I received a cleaning bill for 1700 dollars.  They already took away the amount they received from my insurance coverage of 1000 dollars.  So the bill was now around 700 dollars

It mentioned having to drill out the deadbolt because a few unauthorized tenants where living there and had changed the lock.  They itemized a bunch of cleaning expenses which were ridiculous prices like 100 dollars to remove each item left there.  If it was really true that unauthorized tenants had moved  in, they created this trash....But of course the whole thing could be fabricated by management and are just doing it because I didn't have a walkthrough prior to leaving and can now exploit that.

Seems like it's not my responsibility what happens after I move out when I've told 2 mangers and one maintenance personal that I plan to move out early and they said "ok got it" 

 Anyway I'm wondering what the best thing to do is if I get sent a letter from a collection agency.  Do I explain what really happened and that it's the fault of management to not go to the unit earlier and leave it unoccupied for homeless people to move in, or is it better to say I did a walkthrough the the manager before leaving and gave management the keys etc, so there must be some confusion or they are simply lying in an attempt to get more money from me.  Thanks