Is it obligatory to repay the principal amount along with the mentioned interest, or is it sufficient to repay only the principal amount, to a non-Muslim with whom a Mudarabah (profit-sharing) partnership was formed in the stock market and the money was lost?

The aforementioned Mudarabah (commenda) is invalid because the capital provider stipulated a guarantee for the principal and its profit, and because the profit was determined as a fixed percentage.

If the Mudarabah is invalid, the capital provider is entitled to the entire capital and all profit (if any), and bears the entire loss. The Mudarib (agent) is entitled to a fair wage ( اجر المثل).

However, if the contract was an interest-based loan (Qardh Ribawi), then the lender is entitled to their principal only, without the interest, due to the Almighty's saying: "And if you repent, you may have your principal – [thus] you do not wrong, nor are you wronged."