Is it permissible for one who has the obligatory Hajj to perform a voluntary 'Umrah, and vice versa?

The questioner : All right. And among them is the case where one performs Hajj on behalf of a vow or a voluntary act, while he has a make-up Hajj (qada') for a defective Hajj that occurred, which is due for make-up rather than what he intended, according to the school of thought as well. As for performing a voluntary Hajj after making up the Hajj of Islam, and before performing 'Umrah, or vice versa, is that permissible or not? In al-Talkhis, it is stated that this depends on whether the abandonment is immediate or not. If we say it is immediate, it is not permissible; otherwise, it is permissible, though there is a point of consideration. As for zakah, he said

The Shaykh : that there is a point of consideration in this, and the correct view is that it is permissible for one who has the obligatory Hajj to perform a voluntary 'Umrah, and for one who has the obligatory 'Umrah to perform a voluntary Hajj, because although both are of the same type, being acts of worship, their actions differ and their times differ, so one does not restrict the other.