Is it permissible to work with a university degree obtained by falsifying a high school diploma, with the intention of repenting and studying for the high school diploma later, or is it not permissible to work with it at all?

You must repent to Allah Almighty for forgery and deception, for the Prophet, peace and blessings be upon him, said: "Whoever deceives is not of me." After repentance, there is no harm in working with a university degree that is not forged, provided the work is done proficiently and its rights are fulfilled.