Is it permissible to sell oil to a client who has an exclusive contract with another company before the expiration of his contract, with my knowledge and at a lower price?

The questioner is not permitted to sell the oil to this client, because the client is not permitted to use the tank built for him by the other company, as this violates their condition. If the company only allowed him to use it, then he is obligated to fulfill their condition. However, if they gifted it to him as a conditional donation, then this contract and condition are invalid, and the tank must be returned to the company that built it.