What is the ruling on a man having intercourse with his wife during the period of postpartum bleeding (nifas) if the blood stopped for a day then returned after intercourse?

It is permissible to have intercourse with one's wife on the day of her purity if her postpartum bleeding (nifas) has ceased upon seeing one of the two signs of purity (dryness or white discharge). This is because she is then considered ritually pure. However, if the bleeding returns, she must abstain from all such acts until she completes forty days.