If a husband says to his wife, "You are unlawful to me for a whole month," intending only to abstain from sexual intercourse, does this constitute ẓihār or an oath, and is he obligated to fulfill the entire period?

If a husband pronounces his wife unlawful for him, intending thereby to abstain from conjugal relations with her, this act is reprehensible (makrooh) and does not render her unlawful for him. However, he is required to offer an expiation for a broken oath (kaffarat yameen), regardless of whether he had intercourse with her or not. The details regarding the expiation for a broken oath have been previously explained in Fatwa no. 88445. One must be cautious of marital problems, and in cases of nushuz (disobedience/recalcitrance), there is a Shar'i (Islamically lawful) way to address it.