Connect old credit note to new invoice Xero.
If you want to connect a old credit note with new invoice. First make a invoice when you click on approve button a message will popup to allocate credit. click on Allocate. Enter full amount of credit note or partial amount. click on Allocate credit to submit.
Now credit note balance show in invoice pending.
If you want to add a credit note in unpaid invoice it is easy. Open the Invoice Sales>Invoice click on Invoice Option and select Add credit note.
Now
you can make and add credit note to particular invoice. you can see
full video of making credit note.