Section 24. A city or town, in which the operating agency, pursuant to section forty-seven, proposes to take, acquire or clear land constituting the whole or part or parts of an area which the agency has determined to be a substandard, decadent or blighted open area and for which such agency is preparing an urban renewal plan, may enter into an agreement with the operating agency to bear any loss that may arise as a result of such taking, acquisition or clearance in the event that such land is not used for urban renewal purposes; provided, however, that no city or town shall enter into any agreement under this paragraph which would cause the losses agreed to be borne by such city or town under all agreements under this paragraph in effect at any one time, according to the estimates of costs upon which such agreement or agreements are originally based, to exceed four per cent of its equalized valuation.