Is it permissible to sell goods to a person for him to sell at specified prices, then this person increases the prices for himself, and after a period, gives me a portion of the total value of the goods?

If you authorized a colleague to sell your merchandise, the situation differs:

If it was without a fee: He is not entitled to anything, except that you may please him with something. If it was for a specified fee: He gets his fee, and the surplus is yours. If you did not agree on a specified fee: The agency is invalid, and he is entitled to the customary wage (اجر المثل). If you told him, "Sell it for such-and-such, and whatever exceeds that is yours": The surplus is his.