Is money paid to a merchant for the purpose of trade with the intention of gaining profit after a specified period, knowing that there is a risk in trade and a delay by the merchant in repayment, considered usurious money (riba)? And what should be done if it is: should the amount be taken with the profit, or only the principal, or the principal with the profit given in charity?

If the contract between you and your companion is an investment of money with a guarantee of capital and an additional profit, then this is a corrupt and forbidden Mudarabah (partnership). The contract must be nullified, and all profit is yours, while the worker receives the wage of his like. The distress and turmoil you feel are due to a lack of piety towards Allah and not abiding by His limits. The way out is through piety towards Allah and righteous deeds, for Allah Almighty says: "Whoever does righteousness, whether male or female, while he is a believer - We will surely cause him to live a good life, and We will surely give them their reward [in the Hereafter] according to the best of what they used to do."