What should a merchant do with his partner who did not fulfill his promises and took money from the fund without his knowledge?

A Muslim must fulfill their promise unless it involves disobedience to Allah Almighty, for a promise is a covenant about which a servant will be questioned on the Day of Judgment. The Prophet (peace and blessings be upon him) considered breaking a promise to be one of the traits of hypocrisy. Anyone afflicted with any of these traits must repent to Allah. However, it is not permissible for the one to whom the promise was made to take the promiser's money unjustly; they must return it, along with repenting to Allah. It is not obligatory for them to inform the promiser; rather, they should return the money and conceal their own act.