All praise is due to Allah, and may peace and blessings be upon our Master, the Messenger of Allah.
Upon examining the details of the question, it becomes clear that this arrangement constitutes a contract of ijārah (hire/lease) between the factory owner and the walnut owner, whereby the walnut owner engages the factory owner to roast the walnuts in exchange for a specific wage per kilogram, after which the roasted walnuts are returned to the walnut owner.
It is an established principle that the hand of a hired worker is a hand of trust (yad al-ajīr yad amānah) — meaning he bears no liability except in cases of transgression or negligence.
Based on this, the entire quantity of roasted walnuts produced belongs to the walnut owner, as he is the rightful proprietor. The factory owner has no entitlement to any portion thereof, for he holds it in trust. Accordingly:
It is not permissible to stipulate that the factory owner must guarantee delivery of a specific quantity, as such a condition is jurisprudentially void. Likewise, it is not permissible for him to retain any surplus beyond the agreed quantity, unless the owner expressly permits him to do so. The factory owner's right is confined solely to his agreed wage for the work performed.
Furthermore, stipulating a change in the nature of the hand from one of trust (amānah) to one of guarantee (ḍamān) constitutes a corrupt condition (sharṭ fāsid). According to one position within the Shāfi'ī school, such a condition is rendered void while the underlying contract remains valid.
Imam al-Bujayramī al-Shāfi'ī, may Allah have mercy upon him, stated:
"If one lends something on condition that there is no liability, the loan itself is rendered corrupt — as held by al-Ramlī. It has also been said that only the condition is invalidated, as stated by Sulṭān al-Muzāḥī. Al-Qalyūbī's position is that liability is established even if it was stipulated as a trust, since such a stipulation is a corrupting condition according to the relied-upon opinion." — [Ḥāshiyat al-Bujayramī 'alā Sharḥ al-Manhaj,Vol. 3/P.101]
Accordingly, it is not permissible to stipulate that the factory owner must guarantee the delivery of a specific quantity of walnuts — such a condition is void. He has no right to retain any surplus beyond the agreed quantity unless the owner grants him permission to do so. The contract is one of ijārah, the factory owner's hand is a hand of trust, he bears no liability except in cases of transgression or negligence, and his entitlement is to his wage alone. And Allah Almighty knows best.
