The department shall have the power, in the name of the state, to enter into contracts with any duly authorized representative of the United States government, providing for the admission to, and the separate or joint observation, maintenance, care, treatment and custody in, state hospitals of persons entitled to or requiring the same, at the expense of the United States, and contracts providing for the separate or joint maintenance, care, treatment or custody of such persons hospitalized in the manner provided by law, and to perform such contracts, which contracts shall provide that all payments due the state of Washington from the United States for services rendered under said contracts shall be paid to the department.