Am I obligated to pay the 25% of the capital that the partner lost, knowing that he managed the business himself despite our agreement that I would manage it, and I was warning him about his actions that led to the loss?

If two individuals participate, one with his money and the other with his labor, it is a Mudarabah (definition) partnership (profit-sharing agreement). In this case, the loss is borne by the owner of the capital, and the worker loses his effort and labor, unless he committed an act of transgression or negligence. The condition that the worker shares in the loss is a void condition that does not invalidate the contract. The worker does not guarantee the loss unless his transgression or negligence is proven. The reference point for resolving disputes is experts in the field. If nothing is clear, then the worker's statement, affirmed by his oath, is accepted.