Settle a customer ticket for a customer who already have an outstanding debted amount

Hi Everyone,
I created a customer account and its working fine. but, my problem is that i cant settle his next bill unless i zero the customer account. my intention is to pile his daily bills and settle them later one by one as he submit the payments. at this point after settling the first ticket, i cant settle the next one through customer account since it turns inactive.
please advice on how to rectify this.

Check options on account types, debit/credit etc.