What is the ruling on combining the farewell tawaf with the tawaf of ifadhah, and is it permissible to delay it until the twelfth day?

Al-Huwaini: Is it permissible to combine the tawaf of ifadhah and the farewell tawaf with a single intention, and is it permissible to delay it until the twelfth day?

The Shaykh : What do you mean by the tawaf of ifadhah?
Al-Huwaini: Yes, until the twelfth day.

The Shaykh : This appears to be a question that is frequently asked. There is a principle that an obligatory act does not suffice for another obligatory act, though an obligatory act may suffice for a recommended one. The question now is: can a pilgrim dispense with the farewell tawaf by performing the tawaf of ifadhah? We say that if the farewell tawaf were merely a recommended practice, we would say it is sufficient to intend in one's heart the obligatory act, which is the tawaf of ifadhah, and add to that another intention, which is performing the recommended farewell tawaf. This is on the assumption that the farewell tawaf is a recommended practice. However, the farewell tawaf is an obligatory act commanded by the Prophet (peace and blessings be upon him). He distinguished between it and the tawaf of ifadhah by making the tawaf of ifadhah absolutely necessary for a menstruating woman; if she menstruates, she must delay and not perform tawaf while menstruating, no matter how long her menstruation lasts, until she becomes pure and performs the tawaf of ifadhah in a state of purity. As for the farewell tawaf, the Wise Lawgiver dropped its obligation for a menstruating woman as a concession from her Lord. Therefore, it is not permissible to rely on the tawaf of ifadhah in place of the farewell tawaf, because each is obligatory, and one is more obligatory than the other, namely the tawaf of ifadhah. Muslims often encounter such matters, whether men or women. For example, a woman who owes a ghusl for major ritual impurity then begins to menstruate and remains menstruating for her usual few days, then becomes pure. Is she required to perform one ghusl or two ghusls, one for the major ritual impurity and the other for the menstruation? If we know the principle that an obligatory act does not suffice for another obligatory act, she must perform two ghusls, one for the major ritual impurity.