Is it considered a violation of Sharia if the owner of the capital is not informed of the percentage of profit taken by the intermediary, even if the owner is content with it and the intermediary stipulated beforehand that the percentage should not be disclosed to him?

This transaction is invalid due to the uncertainty of the rent or profit. You have no right to the percentage of profit you are taking without the knowledge of the capital owner. However, you are entitled to a fair wage for the past period. The contract must be rectified for the future. Authorization with an unknown fee is invalid, and one is entitled to a fair wage for it. Mudarabah (profit-sharing) is only valid when the share is known.