(a) make agreements for supplies and commodities needed in the administration of assistance and care, under which local welfare units may, subject to the approval of the department, purchase such supplies and commodities; and
(b) in accordance with requisitions duly approved by the commissioner of general services, purchase as the agent of local welfare units and on their request, supplies or commodities needed in the administration of assistance and care and make payments therefor after making proper deductions from state reimbursements due the local units which avail themselves of the provisions of this subdivision.