Is it necessary to offer expiation for an oath before violating it, or can it be performed after violating it while being aware of the oath, and what are the consequences for one who violates their oath before offering expiation?

The expiation for an oath (kaffarat al-yamin) is not obligatory until after breaking the oath, i.e., after the friend enters his friend's house. It is permissible to offer expiation for an oath before breaking it, according to the majority of scholars, and it is not permissible to delay the expiation without an excuse, according to the majority of scholars.