How to apply a debit memo to a supplier invoice

Hi

I have an invoice for R 10 000 of which R 8000 has been paid the balance of the invoice will not be paid as the client only pays their supplier what is due and not the amount of the invoice. If I raise a debit memo it reduces the amount owing to the supplier which is correct, BUT the invoice amount remains as R 8000 owing and will remain on the aging analysis.

My question is how do I knock the debit memo off against the invoice?

I was hoping to affect a R 0 amount dummy payment which will allow me to select the invoice and the debit memo? BUT only the invoice is available for selection?