What should I do to fulfill the trust of a debt I took from a colleague whom I can no longer reach?

Whoever has money belonging to another for which he is liable must: write a will concerning it, and diligently search for its owner or their heirs to deliver the right to them. If that becomes impossible after searching, then he may donate it as charity on behalf of its owner. If the owner of the right then appears, he is given the choice between affirming the charity, in which case the reward is his, or taking back his money, in which case the reward is for the other person.