QUESTION about new and existing tenants in a college rental

[Max T.]

Single family home rented to 3 Temple students for $1300/month. Tenants are inherited as property was purchased a few months ago. Their lease ends May 31st.

One tenant (AB) wanted to move out and the other two want to stay. I agreed and they found a replacement tenant (ZB) who passed my screening process. The two remaining tenants plus ZB and I signed a new agreement set to begin on June 1st, one day after the original lease with AB expires.

Today (rent is due) one of the remaining tenants tells me that he has sent me an amount (~975) for rent that is less than the $1300 amount, because this is the last month for the tenant moving out AB (who may already have moved out), and because the replacement tenant ZB does not move in until June 1st.

I dont know if this is relevant or not, but AB has not responded to my requests to meet for his final walk through and return of his share of security deposit. 

And just in case this comes up - all tenants are jointly and severally liable for the full rent payment each month according to the lease.

I feel as if I should be receiving the full rental amount and the tenants should work out the exchanging amongst themselves. But alas, how should I phrase that?