What is the Shar’i ruling on someone who spent money entrusted to him to buy a car for another person, then was unable to return it to its owner who traveled and whose news was cut off, knowing that the spender does not possess the money to compensate its owner?

If the money taken was a loan or a deposit for which permission was granted to utilize it, there is no harm in consuming it, and it must be repaid when able. However, if it was a deposit for which permission to utilize it was not granted, or if it was taken without the owner's knowledge and permission, then this is a betrayal of trust and incurs sin. It is obligatory to return it when able.