Is it permissible for one who has made an act of ẓihār against his wife to have intercourse with her during the period of expiation, if it involves fasting for two consecutive months? And would that invalidate the fast and require him to restart?

If a man says to his wife, "You are unlawful to me," the prevalent ruling is that it falls under the حكم اليمين (ruling of an oath), unless he intended divorce or ẓihār. If he intended ẓihār for a temporary period of two months, then he must repent from the ẓihār because it is forbidden. If he has intercourse with her before the expiration of the period, he is liable for the expiation of ẓihār. If he has intercourse with her before performing the expiation, he must repent, but the expiation does not multiply. However, if he has intercourse with her while observing the fast for expiation, then according to the majority of jurists, the continuity of the fast is broken, and he must restart the fast. But according to the Shafi'is, continuity is not broken by intercourse at night.