Is giving fictitious profits to a partner, while currently unable to return the principal, considered usury, especially since this giving of interest was a unilateral decision to manage money?

The hand of a worker in mudarabah (profit-sharing partnership) and a partner in a company is a hand of trust (yad amanah), not of guarantee (daman). Therefore, they do not bear any loss in profit or capital unless there is transgression (ta'addi) or negligence (tafreett). If the money is destroyed without negligence, they are not liable for anything to the owner of the capital.

If the mudarib (working partner) or the partner is unable to repay the money to its owner, the owner of the capital must grant him respite, based on the Almighty's saying: (And if there is one in adversity, then [let there be] a delay until [a time of] ease. But if you remit [it by way of charity], it is better for you, if you only knew.)

Offering what is called "interest" to the owner of the capital in exchange for inability to repay is impermissible, because it falls under the category of a gift to a debtor, which is not permissible in Islamic law, unless it was a custom before the debt. Every gift or benefit that the debtor offers to the creditor because of the debt is considered usury (riba).