If a creditor has not agreed with an obligor to charge the obligor any interest, the creditor may charge and receive from the obligor legal interest at the rate of six percent a year on the principal amount of the credit extended beginning on the 30th day after the date on which the amount is due. If an obligor has agreed to pay to a creditor any compensation that constitutes interest, the obligor is considered to have agreed on the rate produced by the amount of that interest, regardless of whether that rate is stated in the agreement.