[Ruling on One Who Bought from One Partner, Then the Other Demanded the Price of His Share]

Answer—and Allah is the One who grants success: If the partner had given his fellow permission to sell the fruit, then that permission is an agency, and the agent has the right to receive the price. If the agent receives the whole price of the fruit, then there is no recourse against the buyer.
And if the partner had not given his fellow permission, nor appointed him to sell his share, nor—when he learned of the sale—did he ratify it, then the seller has no right to receive the price of his partner’s share, and the buyer has no right to pay it to him; for the partner in that case is an interloper (fuḍūlī), and the right to receive the price does not attach to an interloper. At that point, the partner has the right to demand from the buyer the delivery of the price.
This is when the buyer knows that the seller has a partner; if he did not know that, then let the buyer deliver to the demanding partner his share, and then seek recourse against the seller who took the whole price.
Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2