(c) The purchase price of a project or any part thereof may be payable by a purchase money note only when the cost of the project was financed with a loan or deferred loan by the state. Each purchase money note shall provide for its complete amortization by periodic payments within a period not exceeding forty-one years from its date, shall bear interest at a rate to be determined by the State Bond Commission and shall be secured by a first mortgage on the dwelling unit purchased, provided when the sale is to a tenant of the project or to a cooperative or condominium association, membership in which is open to any tenants of the project or part of the project to be sold, the commissioner may set an interest rate on such purchase money note commensurate with the amount by which the income of any such individual tenant purchaser or of any tenant member of a cooperative or condominium association exceeds the maximum limits permitted for continued occupancy of such project, but in no case shall such interest rate be set below the minimum determined by the State Bond Commission.