Hi there, kmcgoverness.
Thanks for joining on this thread. Allow me to chime in and help you allocate the remaining amounts of the item.
In situations like this, you will only process a refund for the first half of the invoice since they already canceled the service. To account for the remaining amount, create a credit note to keep your record in order.
For future reference, I'm adding a link with detailed information when to apply a credit note or delayed credit.
These steps should point you in the right direction.
Let me know if you have additional questions about QBO. Please know I’m just a comment away for help. Have a good one.