[Ruling on the Muḍārabah Agent Purchasing Half the Merchandise]

Answer—and Allah is the One who grants success and aid: The words of the jurists of the madhhab—as in Sharḥ al-Azhār and its marginalia—indicate that the sale is valid if there is no profit; but if there is profit in the muḍārabah goods, then the sale is not valid, … etc.
Then the marginal note comments with the following wording: “(This wording is general and needs detail. It is to be said: If he sold all of it to the worker, it is not valid with respect to the whole; for he has purchased his own property and the property of another, and that is not valid. As for if he purchased other than his own share of the profit, that is clear—namely, that it is valid … and (establishment)(Í) where he purchased only the owner’s share.” End (establishment)(Í).
“So, if he purchased all of it, it is not valid, because he purchased his own property and the property of another, because ignorance (of shares) accompanies the contract, and ratification is inconceivable here—unless the prices are distinguished.” End. “The preferred view is that it is not valid whether or not the prices are distinguished, because he purchased his own property and the property of another.” End (establishment). (Í)
I say: On this basis, if the sale mentioned in the question pertains to muḍārabah goods while they are free of profit, then the sale is valid; but if profit has already accrued in them, then the sale is invalid.
Yes, the rationale they connected to the invalidity of the sale is that the worker purchased his own property and the property of another.
The proof for the soundness of this rationale is His saying, “Do not consume one another’s wealth unjustly, except it be a trade …” to the end of the verse. [al-Nisāʾ:29] There is no doubt that a person’s buying his own property from another entails consuming wealth unjustly, which is forbidden by His saying, “Do not consume.” On that basis, it is not permissible for the seller to take the price from the buyer.
If it is said: The sale is valid with respect to what the seller owns and invalid with respect to what the buyer owns—so there is no reason for its invalidity in its entirety.
We say: A single contract is not divisible; it is not valid to rule some of the goods validly sold and some invalidly sold while the wording is one, the contract is one, and the thing sold is a single thing—namely, half of the merchandise in the question.
Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2