Effective October 1, 1981, and thereafter, in the case of water and waste disposal and community facility borrowers, and effective November 12, 1983, and thereafter, in the case of housing and farm borrowers, upon request of the borrower, the interest rate charged by the Farmers Home Administration to such borrowers shall be the lower of the rates in effect at either the time of loan approval or loan closing and any Farmers Home Administration grant funds associated with such loans shall be set in amount based on the interest rate in effect at the time of loan approval.