Taking care that enough cash is retained always in the sinking fund to provide for annual payments of principal and interest, the surplus, if any, may be loaned or invested under the direction of the proper governing body as follows:
Investments of the class specified in subsection 2 continue a part of the sinking fund and must be held in custody of the treasurer of the municipality. Bonds representing such investments may be sold by the governing body at any time, but the money received remains, until used, a part of the sinking fund.
N.D.C.C. § 21-03-43