Is it permissible for a fasting person who remembered during intercourse that he is fasting to complete his intercourse and then offer expiation, or must he stop immediately and restrain himself?

There is no expiation for one who has intercourse forgetfully during the day in Ramadan, for the Prophet, peace and blessings be upon him, said: "Lifted from my Ummah are mistakes, forgetfulness, and that which they are coerced into." However, if the fasting person remembers during intercourse, he must stop immediately; otherwise, expiation becomes obligatory due to his intentional act.