What is the expiation (kaffarah) obligatory upon someone who found a sum of money on an airplane, benefited from it, and is unable to return it to its owner or to the authorities?

It is not permissible to take a lost item (Luqata) with the intention of not returning it. Whoever takes it with the intention of betrayal, it is not lawful for him, and he is considered like a usurper (Ghasib). He is obliged to guarantee it to its owner, according to the Prophet's (peace be upon him) saying: "The hand is responsible for what it takes until it returns it."

So, whoever takes it with the intention of possessing it without making an announcement (ta'reef) has committed something forbidden. He is obligated to guarantee it and does not own it even if he were to announce it.

It is obligatory for whoever took it with the intention of betrayal to repent, and to exert effort to reach its owner and return it to him. If he is unable to do so, he should give it as charity on his behalf for good causes.