Is working as an agent for Citibank to market the Mastercard permissible or impermissible, given that the bank offers installment payments with a small interest rate after a grace period?

If the marketed cards comply with Sharia (definition) regulations, there is no harm in remaining in the work. However, if they do not comply, then it is impermissible to market them, as that would constitute assisting in usury. This is based on Allah Almighty’s statement: "And do not cooperate in sin and aggression." And whoever abandons something for the sake of Allah, He will compensate him with something better.