A municipality may issue bonds for the purchase of outstanding special assessment warrants of the municipality before or after their maturity, at the best price obtainable, but not exceeding sixty percent of the par value thereof and the interest accrued to the date of purchase. The rate of interest on the bonds shall not exceed the rate of interest on the special assessment warrants for the purchase of which the bonds are issued, and such bonds shall not be sold for less than the par value thereof plus the interest accrued thereon.