(d) The Legislature shall provide terms and conditions for the Texas Water Development Board to sell, transfer or lease, in whole or in part, any acquired facilities or the right to use such facilities at a price not less than the direct cost of the Board in acquiring same; and the Legislature may provide terms and conditions for the Board to sell any unappropriated public waters of the state that might be stored in such facilities. As a prerequisite to the purchase of such storage or water, the applicant therefore shall have secured a valid permit from the state authorizing the acquisition of such storage facilities or the water impounded therein. The money received from any sale, transfer or lease of facilities shall be used to pay principal and interest on state bonds issued or contractual obligations incurred by the Texas Water Development Board, provided that when moneys are sufficient to pay the full amount of indebtedness then outstanding and the full amount of interest to accrue thereon, any further sums received from the sale, transfer or lease of such facilities shall be deposited and used as provided by law. Money received from the sale of water, which shall include standby service, may be used for the operation and maintenance of acquired facilities, and for the payment of principal and interest on debt incurred.