Upon preliminary approval by the State Bond Commission pursuant to the provisions of section 3-20, the state, acting by and through the Commissioner of Housing, may enter into a contract with a developer, the authority or mortgagor of the authority for state financial assistance in the form of grants-in-aid or deferred loans to housing projects financed by the authority through the means of a loan secured by a first mortgage. Such grants or deferred loans made to a developer or mortgagor of the authority under this section shall be for construction or rehabilitation of developments containing rental units. The total amount of such grants or deferred loans awarded to a single project shall not exceed an amount equal to one-half of the cost of the project divided by the number of rental units in the project multiplied by the number of low-income units in the project. The total number of low-income units in any project receiving financial assistance under this section shall be not less than twenty per cent and shall not be more than forty per cent of the total number of rental units in the project. No project receiving financial assistance under this section shall contain less than twenty-five rental units. Any grant or deferred loan awarded under this section shall be used to reduce the cost of the project. Loan repayments shall be paid to the State Treasurer and deposited in the General Fund.