Section 55. A city, town, or district or regional school district shall not at any one time have on deposit in a bank or trust company or banking company an amount exceeding sixty per cent of the capital and surplus of such bank or trust company or banking company, unless satisfactory security is given to it by such bank or trust company or banking company for such excess. The treasurer of any city, town, district or regional school district shall not deposit funds for which he is accountable in any bank, trust company or banking company with which such treasurer is associated as an officer or employee or has been associated as an officer or employee at any time during the three years immediately preceding the date of any such deposit. For the purpose of paying the principal or interest due on any bond, note or other obligation of the city of Boston, which is payable or requested to be paid in the city of New York, the city of Boston may keep on deposit in any national bank or trust company in the city of New York a sum not exceeding in the aggregate twenty-five thousand dollars; provided, that for a period of two weeks prior to the date of any such payment or payments, said amount may be increased by a sum or sums sufficient to cover the same. A treasurer of a city, town, district or regional school district may invest such portion of revenue cash as he shall deem not required to pay expenses until such cash is available and all or any part of the proceeds from the issue of bonds or notes, prior to their application to the payment of liabilities incurred for the purposes for which the bonds or notes were authorized, in term deposits or certificates of deposit, in trust companies, national banks, savings banks, banking companies or cooperative banks, or in obligations issued or unconditionally guaranteed by the United States government or any agency thereof and having a maturity from date of purchase of one year or less, or in United States government securities or securities of United States government agencies purchased under an agreement with a trust company, national bank or banking company to repurchase at not less than the original purchase price of said securities on a fixed date, not to exceed ninety days or in shares of beneficial interest issued by money market funds registered with the Securities and Exchange Commission under the Investment Company Act of 1940, as amended, operated in accordance with Section 270.2a-7 of Title 17 of the Code of Federal Regulations, that have received the highest possible rating from at least one nationally recognized statistical rating organization and the purchase price of shares of beneficial interest purchased pursuant to this section shall not include any commission that these companies may charge, or in participation units in a combined investment fund under section thirty-eight A of chapter twenty-nine; provided, however, that no temporary notes in anticipation of revenue shall be issued under section four as long as any revenue cash, exclusive of revenue sharing or other revenue cash the use of which is restricted to purposes other than current maintenance expenses, remain so invested.