The Secretary of Defense or his designee is authorized to maintain and operate any housing acquired under this title and assign quarters therein to military and civilian personnel and their dependents. Appropriations for quarters allowances or appropriate allotments, and rental charges to civilian personnel, may be utilized by the military department concerned for the payment of principal, interest, and other obligations, except those of maintenance and operation, of the mortgagor corporation with respect to such housing projects. Such payments shall not exceed an average of $90 a month per housing unit and total payments for all housing so acquired shall not exceed $21,000,000 per month: , That, in case of the United States Coast Guard, total payments for all housing so acquired shall not exceed $90,000 per month.Provided