Such subscription of any county or incorporated city or town shall not become due and payable unless the railroad company shall have constructed and put in operation within the time fixed in the application, and substantially in the direction and on the line as shown in the plan or map, that portion of its railroad located within the county making the subscription, or that portion of its railroad located within the county in which is situated the incorporated city or town making the subscription, as the case may be; provided, that any county, incorporated city, or town making a subscription under these provisions, may, in addition to the restrictions imposed by this section, stipulate with the railroad company that its subscription shall not become due and payable until the company shall construct its road to such points, or for such distances, as may be agreed upon.