What is the ruling regarding the amount of money that was not paid to the university for a sister's tuition fees; should one keep it for oneself, return it to the sister, or give it to the father in lieu of the first academic year's fees, especially given that the sister had offered a reward to whoever could finalize her transaction without paying the fees?

The money you have belongs to the university because you obtained it through deception. It is not permissible for you to spend it on yourself or to return it to your sister. Rather, it must be paid to the university. You can deliver it indirectly if you fear scandal or harm, for the important thing is to return what is due. The Prophet, peace and blessings be upon him, said: "The hand is responsible for what it takes until it returns it."