Return Cash To Unknown
Your concern itself shows sincerity, and that matters a lot in Islam. You did not take the extra money with dishonest intention; in fact, you noticed it, made a note to return it, and tried to trace the customer. This means there was no wrongdoing on your part. When returning the exact person is no longer possible despite genuine effort, Islam does not leave a person trapped in guilt. The principle is simple and fair: responsibility is tied to intention and reasonable effort, not to what becomes impossible afterward. The correct and ethical course now is this:Keep a clear intention that this money does not belong to you. If possible, inform your employer and hand it over to them so they may keep it as unclaimed cash. If that is not feasible or the employer allows you to handle it, then give the amount in charity on behalf of the unknown customer, with the intention that if the person ever comes forward, you will repay them and the reward of charity will belong to them. Islamic accountability is not blind punishment; it is justice.A person is not held sinful for something beyond their control. The Prophet ﷺ taught that deeds are judged by intentions, and Allah does not burden a soul with what it cannot manage. You may also make a simple duʿā, asking Allah to convey the reward to the rightful owner and to clear your conscience. With your intention and action, you have already fulfilled your moral responsibility.