Subject to the provisions of N.C. Gen. Stat. § 25A-23, if all involved parties agree, there may be a substitution of collateral under a security instrument in a consumer credit sale. For such substitution, the seller may impose a fee not to exceed ten percent (10%) of the unpaid balance of the debt or fifteen dollars ($15.00), whichever is less. (1971, c. 796, s. 1.)