Dr. Monzer Kahf, a prominent economist and counselor states: “eating from such provisions depends on what kind of Haram is it. When you talk about Haram that is coupled with Moral shame (in Arabic Shayn) such as steeling, robbery and you know that the item that is offered to you is stolen. When you talk about other Haram like sale contracts that violate the principles of Shari’ah, including Riba, or PRICE of alcohol or pork or income of a prostitute, we have nothing in our Shari’ah that requires us to boycott such people. It is permissible to have any kind of exchange contract with them as long as the contract and its subject item are permissible in Shari’ah, WE ARE NOT REQUIRED TO ASK THE QUESTION WHERE DID YOU GET THIS INCOME FROM, and even when we know it we are not required to abstain from any legitimate relation with people whose income are from such sources. A gift in this regard is like exchange and that includes accepting food offered by them. Simply this is not of our business.
On the other hand, there is the Taqwa factor, of course in this regard, we stay away from being close to the Haram and the Taqwa factor makes us avoid being close to people with income that has Haram elements in it, wholly or partially. But this level of personal behavior is not required in a sense it does not define the boundary between what is permissible and what is not permissible.”