Beginning with the fiscal year 1942, each appropriation to enable the Secretary of Agriculture to carry into effect any program administered through the Production and Marketing Administration may, in the discretion of the Secretary, be reimbursed out of the then current appropriation for the agency affected, for a fair share of the administrative expense, as estimated periodically or in advance by the Production and Marketing Administration of maintaining registers of indebtedness and making, out of such Production and Marketing Administration appropriation, set-offs under the order entered by the Secretary on May 8, 1937, as heretofore or hereafter amended, in favor of any other agency of the Government.