In some situations you want to be able to settle an invoice with a credit note, or with an expense from the same business partner.
Invoices with status 'Final', 'Partially settled', '1st reminder' and '2nd reminder' can be settled with an Expense or an Expense Credit Note from the same contact.
We now give an example of an invoice that we sent to a debtor and for which we also received an expense for a service provided to us.
If we now look at the Expenses overview, we see that we have also received an invoice from the same contact 'Axit International Trading' for services provided to us.
We have communicated with the contact in question and he has agreed to (partially) offsetting our invoice to him against his invoice (expense) to us.
We now open the invoice and press 
We now select under Payment method - Settlement and then we select under Settlement with the relevant expense (in this example 20240000001 a €145,20)
Now press 
The status of the invoice now becomes 'Partially settled' and under Invoice history it is now shown which payments have already been made and what amount still needs to be paid.