What is the ruling on a scholarship student not returning to his country after the completion of his studies, knowing that he received his salary from the university throughout the scholarship period?

If the scholarship-sponsoring entity stipulates your return to your country after the scholarship, or the repayment of what they spent on you if you do not return, then you must fulfill the condition, and it is not permissible to violate it without an excuse. This is based on the Almighty’s saying: "O you who have believed, fulfill [your] contracts," and the Prophet's (peace be upon him) saying: "Muslims are bound by their conditions."