Any payments made by the employer or his insurer to a workman injured or afflicted with an occupational disease, during the period of disability, or to his dependents, which under the provisions of this law, were not due and payable when made, may, subject to the approval of the commission, be deducted from the amount yet owing and to be paid as income benefits; provided, that in case of disability such deduction shall be made by shortening the period during which income benefits must be paid, and not by reducing the amount of the weekly payments.