The parties may agree to add the unpaid balance of a consumer credit transaction not made pursuant to open-end credit to the consumer's open-end credit account with the creditor. The unpaid balance so added is an amount equal to the amount financed determined according to the provisions on finance charge on refinancing, section 2‑504. This section does not apply to consumer leases.   [PL 1973, c. 762, §1 (NEW).]