The answer of the question depends upon what authority 'Z' holds. To be a qualified personnel to decide price of a product manufactured by some other party is very essential.
Since 'X' is producing the wheat, he is well acquainted with the knowledge of how much investment he has done in terms of labor, infrastructure or raw materials, he holds complete right over his products and it should be he who shall decide the price of the product.
But suppose 'X' does not has any reach to customers or markets, he may use service of a third party for marketing of product. In this case, 'Z' can play a role and earn a share in the profit made by 'X'. If 'Z' is a government authority, he may use his right to decide the price of commodity keeping in mind the income that 'X' must generate for his product.
