Is it considered a sin or an obligation to return money if a portion of funds collected for a sick colleague, exceeding his needs, was used to help another needy colleague without informing the donors?

You are an agent for delivering the collected money to the person for whom it was gathered. You do not have the right to dispose of it by deducting a portion of it or spending it on someone else, except with the permission of the owners of the money. You must pay what was deducted from the money to the person for whom it was collected, unless the owners of the money are informed of what you did and approve of it.