The city may be the purchaser at any delinquent sale under this chapter in like manner as it may become the purchaser of property sold for delinquent general city taxes, and when the city becomes the purchaser it shall transfer into the special fund devoted to the improvement the amount of the delinquent assessment, penalties and interest then due thereon. In cases where under the law the city is not always the purchaser at sales for delinquent general city taxes, the city shall become the purchaser at any delinquent sale under this chapter if there is no other purchaser.