Are the funds taken secretly by a partner, without the knowledge of his co-partner, considered a debt upon him Islamically, given his awareness that he does not possess the means to repay these large sums?

The fundamental principle in partnerships is that partners share in profits and losses, each according to their share, unless it is proven that one of the partners committed an overstep or negligence, in which case they are liable for guarantee. The partnership's funds are held in trust and are not guaranteed except in cases of overstep or negligence. Among the oversteps is for one of the partners to take out a loan against the company's funds or to purchase something for which he does not possess the price, without the permission of his partner. In this case, he alone bears the price of what he purchased, and he is entitled to its profit and is liable for its guarantee. He is also liable for the debts he incurred without the permission of his partner, and these debts become established as an obligation upon him. He is then required to sell his properties exceeding his basic needs to repay these debts.