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.
When a partner in a business promises his co-partner at the breaking up of their joint partnership that he will reimburse him one day, this is not considered a loan. It is just a promise.
Islam urges people to fulfill their promise. The good and pious people are known to fulfill their promise.
So, the majority of scholars believe that fulfilling the promise is likable. However, the Malikis believe that it is mandatory to fulfill the promise if the other party will incur any harm, otherwise.
Allah knows best.