Applying "cash" in AR when no cash has been received

We have a customer in AR that will have activity in AR each month.  That activity will be offset by other GL accounts that do not run through AR.  Each month, we will look at the customer balance as well as the balance in the other GL accounts and adjust accordingly.  Some months, we may receive cash from the customer, but many months, we will not receive cash yet the AR balance will go down or be eliminated based on the balance in the other GL accounts. 

We currently do this work using journal entries,but we would like to start using AR to age and keep track of the customer balance.  How can I set up AR so that if we need to decrease the customer balance in AR, but no cash is received, I can do this?