answer:Yes, since you collect the money and commingle it, and then pay the bills out of all the collected and pooled money, you are responsible and liable. Now how to solve the problem: You can pay her back the overages right now. All done, she’s happy, just go on. OR You can tell her, “yes, I owe you that, but since I don’t keep that close of a watch on my total account, I don’t have the extra right now. Let’s work out a solution.” And then you can figure out how to fix it. You don’t say how much she overpaid. A couple ideas though are to have her deduct it all from next months total, or if that is too big a hit, over the next, say, three month’s totals. You need to determine how much you can cover each month, and from that see if you can work out a similar time for her. It maybe a bit painful to be real close on your money for a couple months, yet it will work out best in the end.