Is it permissible to purchase a forged high school diploma for university admission, and what is the ruling on continuing one's studies using it?

What you did by purchasing a German high school diploma is cheating and forgery, and you must repent from it, because the Prophet, peace and blessings be upon him, said: (Whoever cheats is not of me), and his saying: (Beware, and false speech, and false witness). As for completing your studies, there is no harm in it along with repentance. The son is not obligated to obey his parents in choosing a specific major or studying abroad, so if you do not find enthusiasm for studying or if it is difficult, return to your country.