I reside in a European country that has a social security system, which is compulsory health insurance covering 70% of treatment costs. I ask about the ruling on this insurance, and what should a Muslim do if he receives more from it than he paid?

All praise is due to Allah, and peace and blessings be upon the Messenger of Allah, and upon his family and companions. There is no harm in compulsory insurance, but it is not permissible to benefit from it beyond the amount paid. The ruling on insurance does not differ based on the location, but rather on the circumstance. The compelled person is not the same as the one who chooses. It is not permissible for a Muslim to insure himself against illness, whether in Muslim countries or in non-Muslim countries, due to the presence of gharar (definition) (excessive uncertainty) and gambling therein. And Allah knows best.