Interest upon the loan or forbearance of money, goods or things in action shall be at the rate of twelve percent per annum for the period commencing on March 19, 1980, through August 31, 1983, and at the rate of six percent per annum commencing on September 1, 1983, on the unpaid principal balance, unless a greater rate, not exceeding the rate of interest provided in section 45-101.03, be contracted for by the parties.