Funds appropriated for the rental assistance program established by sections forty-two to forty-four, inclusive, or which may become available therefor from the federal government or any other sources, shall be allocated within the following limits:— cities with over five hundred thousand population, not in excess of fifty per cent of such funds for any one such city; cities and towns with between one hundred thousand and five hundred thousand population, not in excess of twenty per cent of such funds for any one such city or town; and cities and towns under one hundred thousand population, not in excess of ten per cent of such funds for any one such city or town. The department shall allocate funds on the basis of applications therefor from the housing authorities; provided, however, that if, after May the thirty-first, nineteen hundred and seventy, a housing authority has expended all of the funds allocated pursuant to this paragraph, the department may, in its discretion, utilize funds appropriated but not allocated pursuant to sections forty-two to forty-four, inclusive of this chapter to provide additional rental assistance funds to such an authority. The department may make advances of funds to a local housing authority in such amount not to exceed five thousand dollars to an authority in each calendar year for purposes of negotiating leases for the rental assistance program; provided, however, such advances of funds shall be made upon the condition that such advances of funds shall be repaid out of any monies which become available to such authority for said rental assistance program.