Tenant trying to use security deposit as rent payment.

[Chootar Laal]

My tenant for the past few months will be leaving in 2 months and I already have his last month's rent but he is also trying to use the security deposit (which is equal to a months rent) as the 2nd the last month's rent.
I think he is trying to do this to avoid any cleaning/damage charges I may deduct. I dont think he will go to the extent of damaging walls intentionally but I would feel better if I had a security deposit in case he did.
What can I do to enforce payment and not apply the security deposit towards the rent? He is supposed to vacate by Aug 31st.