Upon making such collection upon any such subdivision, the finance director is authorized to note such collection upon the assessment record, give receipt for such certified portion of the assessment as and for the assessment levied upon and due upon the subdivision; provided that this section shall not authorize segregation of any assessment which has been delinquent for a period of two years or more; or in case where it appears that such property as segregated, in the discretion of the city council, is not sufficient security for the payment of the assessment. (1957 code § 12.48.020.)