The City, by Ordinance, may grant a refundable or nonrefundable credit against its tax on income to a taxpayer for the purpose of fostering job retention in the City. If a credit is granted under this section, it shall be measured as a percentage of the income tax revenue the City derives from the retained employees of the taxpayer, and shall be for a term not exceeding fifteen years. Before the City passes an ordinance allowing such a credit, the City and the taxpayer shall enter into an agreement specifying all the conditions of the credit.