Is the salary earned after graduation and study lawful? And is expiation required from one who lied in order to transfer to another university major after the specified transfer period had ended?

The question is unclear, but if you lied to the college's registration department to get enrolled in another major, and you achieved what you desired, then you must repent to Allah for lying. There is no expiation required other than that. There is no impediment to continuing your studies in the major you transferred to, and if you succeed and graduate, there is no harm in working with that degree, and there is no sin in the salary if you perform your work in the best possible manner.