Does seeking to change the agency of foreign products – which have agents in Arab countries – constitute selling over one’s brother’s sale, knowing that the agent may not give importance to these products due to low profit or lack of experience?

Commercial agencies in which the agent receives a fee are considered a form of ijara (leasing/hiring), which is akin to a sale. As long as the two parties are within the term of the agency, seeking to change the agent falls under the prohibition of "selling over the sale of one's brother."