If two Muslims are in a business partnership and one decides to pull out and accept the loss of his investment, but the other promises to return the investment at a time in the future (Allah willing), is this considered a debt?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions. Partnership contract is among the valid contracts. Each of the two parties has the right of cancellation whenever he wants to do so. This is the basic rule. As for the consequences of this cancellation, there are many details to be considered: For example, whatever the amount in cash is both parties would distribute it between them. And the assets that are not in form of cash, they may either sell them or divide them in two shares them as convenes to both of them, since this is their own property. So, they can act in their property by their free will. If selling or distributing may cause any harm to one of them then it is prohibited. The Prophet (Sallallahu Alaihi wa Sallam) said: there should be no harm nor reciprocal harm . [ Ahmad ]. All the above details were pertaining to the cancellation of the partnership. As for the money that is promised to be given (as mentioned in the question) it is not considered a debt and the promisor will not be obliged to pay it. Allah knows best.