What is the expiation for using part of the trust money designated for assisting colleagues and then returning it?

Atonement for that would be by returning the money that was used without the owners' permission, along with repentance and seeking forgiveness. It is not permissible to dispose of trusts and deposits without the permission of their owners. Whoever does so is sinful and transgressing, and sin is lifted by repentance, while transgression is removed by fulfilling the obligation.