No lender shall collect or receive any prepayment penalty on loans secured by real property comprised of one- to four-family residential dwelling units if the prepayment results from the enforcement of the right to call the loan upon the sale of the real property that secures the loan. If the loan is prepaid because of sale to a person who the lender has refused to approve for purposes of assuming the loan or failed to approve within 15 days after receipt by it of written request for approval, the prepayment shall be presumed to result from enforcement of the right to call the loan.