He steals customers of other companies by offering a lower price
6-2-2013 | IslamWeb
Question:
Assalamu alaikum Mufthi Sab, i working in a company which deals in waste management as a marketing executive. is it permissible for me to break a business contract which is already made with a company for a same field by giving them low rate of price. E.x. I h ake an appointment and visit a customer and introduce our company and discuss the rates with them, but already he has a company which they deal in same products and they will cancel their contract if i give low rate. so now i am cancelling so many contracts and entering to the business is it halal to earn in such kind of way?
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, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
‘Abdullaah ibn ‘Umar 
narrated that the Prophet 
said: “No Muslim should sell against the sale of his Muslim brother (i.e. no one should enter into a transaction which his brother has already concluded).” [Al-Bukhari and Muslim]
Hiring is a selling of benefits, and the Hadeeth is evidence that it is forbidden to sell something to someone else while one had already agreed to sell to another Muslim brother; the same thing applies to hiring a person who has already been hired by someone else. Shaykh Ibn Taymiyyah 
said: “What is the same like selling with greater reason is to hire someone while that person has already been hired by someone else. The harm of this hiring is more than the harm of selling in general. What is worse than this is if such a (hired) person holds a position of responsibility [as an executive], or a place that he resides in or from which he gains his living, and then someone else asks to take his place.” [End of quote]
Of course this prohibition applies as long as the two contracting parties are not in the period in which the contractor has the choice to cancel the contract, which is the Khiyaar (choice to cancel the contract) period; as stated by Al-Buhooti in Kashshaaf Al-Qinaa’. However, if there is no Khiyaar period, and the contract became effective between the client and the company on a specified job, then can someone offer a lesser price and lure him to leave the company with which he had already signed the contract, or that he is not prevented from this because the hiring contract is a binding contract and it is not permissible for either party to invalidate it prior to the expiration of the agreed-upon period except with the consent of the other party - and therefore this luring does not have any impact on the contract?
Perhaps the most preponderant opinion as stated by Shaykh Ibn Taymiyyah is that this is prohibited during the Khiyaar period and after it. He said in Majmoo’ Al-Fataawa: “... If either party cannot invalidate it (the contract), then someone may come to him and say to him: “Invalidate this sale and I will sell to you (at a lower price)”; so he would urge him to invalidate the first (contract), and insist on him to invalidate it, so he would accept reluctantly, as is the fact in many cases; that is if he did not even cheat him in a way that would necessitate the invalidation of the sale, and this may be more forbidden...” [End of quote]
Accordingly: the view that it is absolutely forbidden is more appropriate for the reasons mentioned above, and because this act could cause a kind of enmity and hatred in people’s hearts toward each other; even if one of them renounces the contract by his own will, he may hate the one who caused the change of the second party toward him and lured him to invalidate the contract and cancel it. Therefore, it is not permissible for you to lure the customers of other companies to cancel the contracts which they had signed with those companies so that they would move to your company.
However, if you do so, you are sinful but this does not affect your work or the salary which you take because it is neither your essential work nor something intended in it, rather it is something accidental or which you did by yourself. This ruling applies unless your very work for which you signed the contract with your employer is to incite the customers of other companies to cancel their contracts with them, in which case, that kind of work is forbidden based on the view favored by Shaykh Ibn Taymiyyah and thus, the salary from forbidden work is also forbidden.
Allaah Knows best.