Section 26A. The county commissioners of the county wherein any public way is to be repaired or improved under the provisions of the preceding section may contribute and expend county funds therefor in accordance with such agreements as the commissioners may make with the department and the selectmen of the town. Said county funds may be paid to the department or to the town from time to time as the work progresses, to the extent that the said commissioners are satisfied that the work for which agreements have been made is being done in accordance therewith. Such contributions or expenditures by a county shall not render it liable for defects in any way or for damages to persons traveling thereon, and when the work of repair or maintenance for which such contribution or expenditure is made is completed, there shall be no further obligation on the part of the county as to the repair and maintenance thereof until a further contribution is made by the county commissioners for such purpose.