how to record a short payment of invoice that pays for a purchase

I have a client that short pays our invoice each month to cover their invoice for a service they provide to us.

I enter the purchase and the sale for correct GST however there is alwasy short payment and am not sure how to apply the short payment of invoice against the relevant purchase.

I am left with an outstanding balance on our sales that matches the outstanding purchase amounts from that client.

Is there any way to overcome this they wont pay the full amount on invoice and let me pay them :(