[Sale in Customary Partnership without Prior Permission]

Answer—and Allah is the One who grants success: If the situation is as described in the question, then the sale is valid for the whole, and the partner’s permission or approval is not a condition. That is because the customary partnership is like a mufāwaḍah partnership, in that each one of the two (or more) partners is authorized by the other to sell, buy, and dispose according to what the interest requires.
They have said, concerning mufāwaḍah (partnership): each of the two partners has what his fellow has and bears what he bears—absolutely; that is, whether or not one of them knows of the other’s disposition.
And in the marginalia of Sharḥ al-Azhār regarding the customary partnership: what one of the partners acquires is for all of them and upon all of them—even if he attributes it to himself. And in it: This is what the fatwas of our master al-Mutawakkil ʿalā Allāh ran upon; it is what the later scholars acted upon; and it is what custom established.
And in another marginal note after this: As for what custom has established, upon which the fatwa and practice ran: what one partner acquires for himself is [nonetheless] for all of them and upon all of them. End of dictation by our master Ḥasan al-Shabībī. End (established). (Í)1
I say: The rulings of the customary partnership are built upon custom. Customs have run that what one of the two partners purchases is for all of them, and what he sells is binding upon all of them.
Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2