You can "pay selected items" only, if you goto debts & select the customer you need to pay you will get the "payment assistant" window click the "pay selected items" button you will then see a list of all outstanding invoices for that customer you can either pay oldest by clicking the "pay oldests" button or if you double click the item you wish to pay in the list you will get another window open with that transaction details , check amounts are correct the click save
job done
NB : If you need to pay more than one just double click a second item and repeat