What should I do about a small debt that I was unable to repay to its owner, who abandoned the transaction, and how can I act without causing embarrassment?

You must return the debt to its owner as long as you are able, or seek his forgiveness, for the Prophet, peace and blessings be upon him, said: "The hand is responsible for what it has taken until it returns it." If you are uncomfortable returning it directly, you can deliver it to him through another means that clears your conscience, such as sending it with someone else or depositing it into his account. It is preferable to inform him of it. If the creditor forgives the debt willingly, then your conscience is clear, and it is better to reward him with supplication or otherwise, for the Prophet, peace and blessings be upon him, said: "Whoever does a kindness to you, then reward him. If you do not find anything to reward him with, then supplicate for him."