What is the Shar'i evidence that ill-gotten gains spent by a person before repentance remain his responsibility, and he must return their equivalent upon repentance, taking into consideration the rule "Repentance erases what came before it"?

Dr. Abbas Al-Baz spoke about the ruling on a poor person and a non-poor person benefiting from unlawful wealth if they possess it and its owner is unknown. He mentioned that Al-Harith Al-Muhasibi does not permit a poor person to benefit from it, while the majority of jurists do permit it, and he favored the opinion of the majority.

Dr. Al-Baz differentiated between a poor person and a non-poor person, stating that a non-poor person would be transgressing by spending unlawful wealth because there is no justification for them to do so. If they repent, they must clear their conscience by returning the wealth to its owner if known, or by giving it as charity if unknown.

This issue is contentious, and there is an opinion by Sheikh Al-Islam Ibn Taymiyyah who believes that a person who repents from unlawful wealth may keep it and it becomes lawful for them after repentance, and they are not obliged to dispose of what they possess of it. He based his argument on the Almighty's saying: ﴿So whoever receives an admonition from his Lord and stops, then he may keep what he has already earned, and his affair is for Allah.﴾ [Al-Baqarah: 275]. There is no harm in following this opinion for whoever believes in its correctness and finds it more compelling.