In any case where the required public areas, in addition to public streets, exceed five percent of the total gross acreage controlled by one owner, then the Planning Commission, with the concurrence of Council, may require such areas in excess of the five percent to be reserved for a period not to exceed two years from the date of recording of the final plat. The sale price for such a required public area shall be agreed to by the subdivider and the City or the Board of Education at the time of the approval of the preliminary plat, such price to be held firm for the two-year period. If, at the end of such period, neither the City nor the Board of Education has either entered into a contract to purchase such reserved land or has instituted condemnation proceedings therefor, then the subdivider shall be no longer bound by the reservation and all such land may be developed by the subdivider in accordance with these Subdivision Regulations. However, the lot size and plat restrictions shall be at least equal to those of the remainder of the plat.