Questionable extra job
2-3-2004 | IslamWeb
Question:
I am student at the UK. My company has sent me for long term training here in UK and they give me monthly allowance which is more than enough to me and my family. But in order to increase my income and help my family back home and also to improve my language I started working at Home Base shop where we sell Home and Gardening stuff. One of the payment methods that this company uses is that the customer can buy the goods and pay nothing till one year or six months but if he could not pay by that time the company will charge him 25% extra. Is it Halal for me to work at this place? Only very few people use this way of payment and my job is not involved in any of the procedure of this agreement.
Answer:
All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad 
is His slave and Messenger.
It is not permissible to work in any institution which deals with interest and usury. The worker in such institution is either dealing with transactions which have interest and usury or is helping another person who is doing so. All of this is unlawful. Jabir 
narrated that the Prophet said: “Allaah has cursed the one who consumes Riba, the one who gives it to others, the one who records it and the one who witnesses it (its transaction).” He said: “All of them are equal in sin.” [Muslim and others]
Also, helping in sin and disobedience is forbidden according to the texts of the Quran. Allaah Says (what means): {…Help you one another in AlBirr and AtTaqwa (virtue, righteousness and piety); but do not help one another in sin and transgression….} [Quran5:2] What you have mentioned about the dealings and transactions of the company is really the interest and usury of the time of the Pre-Islamic Era of Ignorance.
Imam Maalik 
reported in Al-Muwatta’ that Zayd Ibn Aslam narrated that ‘At the time of Jahiliya (Pre-Islamic Era of Ignorance) people used to say to the indebted person as soon as their debt became due: “Either you pay the loan or you accept Riba (usury and interest)”. Then he had to pay or they would increase the amount and extend the period’.
Therefore, you must stay away from this institution even if your work does not deal with usury and interest transactions or dealings.
Allaah Knows best.