Whenever the municipal governing body deems it to be in the best interest of the municipality to waive or cancel any part or portion of the penalty or penalties belonging to the municipality from delinquent special assessments, the governing body by resolution may waive or cancel such part or portion of the penalty or penalties; except that any penalties which have been set aside by law for the purpose of paying bonds issued in the respective districts, together with the interest thereon which has accrued upon any delinquent special assessment or installment thereof, or for the purpose of securing bonds or warrants for street improvement purposes or sewers, may not be waived or cancelled by the governing body.