What is the ruling regarding a man saying to his wife: "This act is forbidden to me," intending thereby sexual intercourse? And what is the ruling regarding his fondling her after five days? And what is the expiation for that?

Swearing by the unlawful (al-haram) to abstain from conjugal relations with one's wife is an unlawful ila' (vow of abstinence) if it is perpetual or for more than four months. This oath does not prevent you from having intercourse with your wife. Rather, if you do have intercourse with her, it necessitates an expiation for breaking an oath (kaffarat yamin). As for foreplay with one's wife until ejaculation, it requires an expiation for breaking an oath, unless you intended by your oath specifically to abstain from vaginal intercourse.