Charge back on a returned item that's already been refunded

Hi all,

Looking for some advise here please.
I’ll try and keep it short and sweet and not bore you to death.

Customer bought an item from us, 20 days later they opened a returns request stating item no longer needed, no problem our end, we accepted the return.

Once the item got back to us it had been opened used sufficiently and the retail box it comes in covered head to tow in brown masking tape, as we couldn’t sell this on again as new and have been used for possibly those 20 days, we issued a partial refund.

I’ve today seen that the customer also opened a charge back on the 21st last week ( we never got an notification of this via email) and it states we have until the 1st of may to respond, we can’t respond as it doesn’t allow us to, it says it’s processing and we can see that the money has been held on our payments page and it has added a defect to our account, so the customer will get the partial refund plus the original amount, can anyone advise as to what we can do in this instance, Usually we always get an email notification in the event of an A to Z, however we’ve had nothing in regarding the charge back, it’s simply because i checked our account health today that I’ve noticed it.

Thank you