The state, acting by and through the Commissioner of Housing, may enter into a contract with the authority, developer, or mortgagor of the authority and the authority may enter into a contract with a developer or mortgagor of the authority to provide state financial assistance in the form of rental subsidy certificates for each low-income unit in the project. Any commitment to provide such subsidy shall be an obligation of the state or the authority, as the case may be, for a period of not less than fifteen years, and the amount of such subsidy shall be equal to the difference between the amount of rent plus an allowance for heat and utilities not included in the rent approved by the commissioner or the authority, as the case may be, and thirty per cent of the annual aggregate family income of the tenant residing in the low-income unit for each such unit on an annual basis. The rent charged for a low-income unit may not be increased without the approval of the commissioner or the authority, as the case may be. The annual aggregate family income of a tenant for the year prior to the occupancy of a low-income unit by the tenant shall not exceed fifty per cent of the area median income, adjusted for family size, as determined by the commissioner or the authority, as the case may be. If such annual aggregate family income after occupancy exceeds seventy per cent of the area median income, adjusted for family size, the unit occupied by the tenant will no longer be considered a low-income unit and the next available unit will be rented to a tenant with an aggregate family income of less than fifty per cent of the area median income, adjusted for family size. No tenant residing in a project will receive financial assistance through a rental subsidy certificate under this section if the aggregate family income of the tenant in the prior year exceeds sixty per cent of the area median income, adjusted for family size.