In ILLiad, you can mark an item paid through the Billing Manager or in the ILLiad Client. If you are billing through IFM, then you will see it on your monthly statement. In ILLiad, see:
Tracking Payment of Lending Invoices
If you are billing in the Billing Manager, see:
If you are billing in the ILLiad Client, see:
Tracking Payment of Lending Invoices