Is it permissible for a person to take a percentage from the profit of a land he purchased for his partner, manage its buying and selling, and guarantee its capital, without his partner's knowledge, under the pretext that informing him might anger him?

You are an agent for your friend in this transaction, and an agent is a trustee. Therefore, he is obligated to act in the best interest of his principal. He is only liable in cases of transgression or negligence. Thus, stipulating a guarantee for the capital and taking money in exchange for this guarantee is not permissible, because agency is a trust and does not become guaranteed by a condition. As for taking a percentage in exchange for managing buying and selling, it is not permissible except with your friend's knowledge. If you do that voluntarily, you are not entitled to anything, and if it is for a fee, then the person from whom you take it must be aware.