A more thorough explanation:

Definition: A due-bill is a written acknowledgement of a debt. It is also known as an IOU, which stands for "I owe you".

Examples:

• John borrowed $50 from his friend and wrote an IOU to repay the debt.
• Mary received a due-bill from her employer for the bonus she earned.

The first example illustrates how John acknowledged his debt to his friend by writing an IOU. The second example shows how Mary's employer acknowledged the bonus owed to her by issuing a due-bill.