The city shall have the power to sell the supplemental assessment and any bonds issued to represent the unpaid supplemental assessments (if the legislative body orders the issuance of said bonds) in such manner as the legislative body directs; provided, however, that the interest rate on the bonds (if ordered to be issued) shall not exceed 6 percent per year. The city shall also have the power to hold said unpaid supplemental assessments or bonds, and enforce the lien thereof, in the same manner as a private owner. Said supplemental assessments, bonds or the proceeds of the sale thereof may be used for the repayment of advances theretofore made by the city in the matter of the acquisition.