What is the ruling on my inability to return money to my Christian colleague, who bought some items for me because I did not have "small change," and whose news was then cut off?

If you are able to contact the person to hand over the remaining amount, or send it with a trustworthy person, that is good. If you expect to meet him later, then wait to hand over his right to him. If you are unable to do so and lose contact with him, then he is treated as if he were deceased; if he has Muslim heirs, the amount is handed over to them. If he has no heirs, the money is disbursed to the treasury of the Muslims (Bayt al-Mal), because it is money for which there is no rightful claimant, similar to the money of a Muslim who has no heir.