Is it permissible for me to trade with money entrusted to me, while guaranteeing the principal in case of loss, and obtaining a percentage of the profits while returning the remainder to the original capital?

This covenant is a trust (amanah (definition)), and it is not permissible for the trustee (mu'taman) to dispose of it in a way not authorized by its owner or required for its preservation. Otherwise, he is sinful and liable for its loss. The fact that he guarantees the amount does not permit him to dispose of it. He must seek forgiveness and repent if he has done so. As for the profit, it is closer to the objectives of Islamic law (Maqasid al-Sharia (definition)) for it to be shared between the owner of the trust and the one who traded with it, and it is better to return all the profit.