What is meant by the maxim is liability of the hand, not of the contract.
Hence, whoever picks up a lost article intending to appropriate it for himself, his hand is deemed a usurping, liable hand until he restores the item to its owner.

Twelfth Maxim: “There is no scope for ijtihad where an explicit text exists.”
Lexical meaning: The Arabs say, “The drink sāgha in the throat,” when it descends smoothly because the passage is open; therefore “lā masāgh” means “no passage, no way.”
Ijtihad is the scholarly exertion made to derive legal rulings from their evidences, and it is of two kinds:

1. Ijtihad in understanding the texts so that they can be applied; this is obligatory upon every mujtahid.
2. Ijtihad based on analogy and considered opinion; one may not resort to this until no ruling for the matter under study is found in the Qur’an, the Sunnah, or consensus—and this is the type intended here.
   An ijtihad is not overturned by another ijtihad: Abū Bakr gave judgments in issues where ʿUmar held a different view, yet when ʿUmar assumed authority he did not revoke Abū Bakr’s rulings.
   Similarly, if a scholar exerts ijtihad on a question and acts upon it, then later adopts a different view and abandons the first, his second ijtihad does not undo the ruling that issued from his first.

Thirteenth Maxim: “Whatever an obligation cannot be completed without is itself obligatory.”
Scholars say this is a principle of legal theory rather than a merely jurisprudential rule.
It is part of the broader maxim: “The means take the rulings of their ends.”
Thus the means to an end assume its legal status: the means to obligatory acts are obligatory; the means to forbidden acts are forbidden; the means to recommended acts are recommended; the means to disliked acts are disliked; and the means to permissible acts are permissible.
For when Allah commands something, He implicitly commands whatever is indispensable for its fulfillment; and when He forbids an act, He likewise forbids the means that lead to it, and so forth.