Section 131G. All vouchers submitted by a vendor under the medical assistance program authorized by section one hundred and thirty-one E shall be signed under the penalties of perjury; provided, however, that an institution, as defined in clause (d) of section two of chapter one hundred and eighteen E, may, in lieu of this requirement, agree in writing with the commissioner that its books and records will be available for inspection at all reasonable times by the commissioner with respect to services rendered under said assistance program.