Is selling goods to a merchant who supplies oil companies, knowing that the merchant only obtained the supply contracts by paying bribes, considered impermissible (haram)?

The forbidden bribe is that which is given to invalidate a right or to establish a falsehood. As for what is paid to attain a right or a deserved benefit that cannot be obtained except by paying something, there is no sin upon the payer (the briber), but rather the sin is upon the recipient (the bribed). If the merchant pays officials to obtain a legitimate right, such as winning a bid according to its conditions, then there is no sin upon him, and the sin is upon the officials. In this case, dealing with him is permissible. However, if he pays to infringe upon the rights of other merchants, then it is a forbidden bribe for both parties. It is more cautious for your friend not to deal with this merchant due to the suspicion of assisting him in falsehood.