Is the student sinful for taking the bus driver's money if she did not pay him for the days she did not use the bus, knowing that he did not stipulate payment in advance, and she believes that she did not benefit from the service on those days, while the driver believes he is entitled to payment?

If a person agrees with a bus owner to transport him to the university without specifying the fare or the duration, this is a فسخ (voidable/corrupt) rental contract because knowing the fare and duration is a condition for its validity. In this case, the bus owner is entitled to a fair wage (اجرة المثل), and this wage is determined by referring to the prevailing custom in such a situation. However, if no agreement was made between the two parties at all, then the bus owner is only entitled to a fair wage.