What is the ruling of the Sharia regarding demanding the return of money to its owner after it was confiscated by the authorities in another country, knowing that the person failed to declare it at the airport due to an oversight, and is facing charges of money laundering?

What transpired between you and your friend is an agency agreement for purchase. An agent is trustworthy and is not liable except in cases of transgression or negligence. There is no difference in this regard whether the agent works for a wage or volunteers. The question remains whether your inattentiveness to the documents constitutes negligence or not. This goes back to the assessment of experts at the airport. If they deem it negligence, then you are liable for the money; otherwise, you are not. In the event of liability, you may reconcile with him to waive a portion of the money.