Русско-англо-русский юридический словарь

Debts in arrears are often sold on to businesses that specialise in chasing overdue repayments, meaning that the demand for payment scales up accordingly. These businesses buy a book of overdue debt for less than its face value and then chase the debt – making a profit if they succeed in securing full repayment. (Independent)

Let us assume that a supplier has sent you goods giving you 30 days credit. Till the 30 days are up, the amount is outstanding but not overdue. After 30 days the amount is outstanding and is overdue.

‘Overdue’ is sometimes called ‘past due'. (Quora)

To put it simply, a debt becomes past due when the payment date comes and goes but you haven’t paid. The past due balance is the amount that was owed by the original due date. (Bond Street)