Did the truthful one break his oath, and is expiation due upon him after he swore to the mechanic that he would take the payment, and then the mechanic threw it into his car?

Whoever swears to do something and then acts contrary to it has broken his oath, and atonement becomes obligatory upon him. Your friend breaks his oath if he had no intention, and if the mechanic did not take the payment. Merely taking the payment and throwing it into the car is not sufficient to fulfill his oath. The atonement for an oath is feeding ten poor people, or clothing them, or emancipating a slave. If he cannot find any of these, then he must fast for three days.

However, if his intention was merely to take or seize it, even if the recipient did not benefit from it, then he does not break his oath. Similarly, if his intention was to give the payment to the mechanic as a gesture of generosity, not as an obligation, then no atonement is due upon him.