Section 28. If a town in which a town way or private way has been laid out, relocated, altered or approved in pursuance of the two preceding sections does not make and complete the same in the manner prescribed by the county commissioners, and to their satisfaction, within six months after it has been laid out, relocated, altered or approved, or within the time directed by them, they shall, forthwith, cause such way to be completed, and the expenses, interest and charges thereof shall be determined and paid in the manner provided in sections fourteen and fifteen.