(a) Loans made by each credit union shall bear interest at a rate or rates as may be determined by the credit union's board of directors. A board may delegate all or part of its power to determine the interest rates on any lending transactions. The board may also authorize a refund of interest on loans under the conditions it may prescribe.(b) A loan may provide for variable interest rates, so long as the factor or index governing the extent of the variation is not under the control of the credit union and can be readily ascertained from sources available to the public or any other index approved in writing by the commissioner which is not available to the public.