[Permissibility of selling a waqf]

Answer—and Allah is the One who grants success: I have seen in the words of Imām al-Mahdī Muḥammad b. al-Qāsim al-Ḥusaynī al-Hūthī—may Allah have mercy on him—that this is permissible; and the jurists of the madhhab said: It is permissible to sell [part of] a waqf to repair another part of it.
Yes: Evidence for this may be drawn from the established principle that necessity has exceptional rulings. If leaving the mosque unrepaired would lead worshippers to abandon it, or if leaving it unrepaired would lead to successive ruin and collapse such that it would later require multiples in cost to repair—and the guardian is unable to arrange a loan on behalf of the mosque or to pledge (mortgage) a waqf asset, and none of that is feasible—then there is no harm in selling some of the waqf in that circumstance.
Also, in the waqf of the Commander of the Faithful ʿAlī (Peace be Upon Him) is that which indicates the permissibility of selling part of a waqf to repair another part.
And the Messenger of Allah (May Allah bless him and his family and grant them peace) ordered, in some narrations, the sale of a mudabbar slave due to his master’s need for the price.

Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2