Increase in Debt Due to Delay

Question
What is the ruling on the penalty for delay imposed by the creditor on the debtor in case of delay in repaying the debt within the specified period? The scenario is as follows: A sister bought a mobile phone on installment for (300) dinars, agreeing to pay (100) dinars each month. If she delays in paying the installment on time each month, she is required to pay a delay penalty of ten dinars. What is the ruling?
Answer
I say, and with God's help: This is a type of usury; because the penalty clause in debts is usury, and God knows best.
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