My son is an employed to canvass and sell credit cards. Is this Haram? Will it be OK for him to continue the job if he tells the Muslim customers that after a fixed period of time the credit will be subject to an additional charge of interest?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

It is not permissible for your son to work in this field, whether he informs the Muslims about it or not. These cards  are a means of Riba (interest), and the Prophet (Sallallahu Alaihi wa Sallam) cursed the one who consumes Riba, the one who gives it to others, the one who writes it down, and the one who witness it. He said: "All of them are equal in sin."

It is also not permissible to sell them to non-Muslims as they are also addressed to deal with the branches of Shariah in transactions (dealings) by consensus, as reported by Al-Taftazani stated in Al-Talweeh Sharh Al-Tawdheeh.'

Allah knows best.