Khiyaru ash-shart ( خيار الشرط) is  Choice of stipulation, Optional condition where one of the parties stipulates for a period of time or days.
The scholars have disagreed on the validity of the option of condition in exchange, leading to two opinions:
The First Opinion: The option of condition in exchange is not valid, which is the consensus of the four legal schools: Hanafi [488] - "Tabyīn al-Ḥaqāʾiq" by al-Zaylaʿī (4/136); "al-ʿInāya" by al-Bābartī (7/138). , Maliki [489] - See: "al-Tāj wal-Ikleel" by al-Mawwāq (4/309); "Mawāhib al-Jaleel" by al-Hattāb (6/137). , Shafi'i [490] -"Fatḥ al-ʿAzīz"by al-Rāfiʿī (8/314), (al-Majmūʿ), al-Nawawī (9/192). , and Hanbali [491] -"Kashshāf al-Qināʿ"by al-Buhūtī (3/204); "Maṭālib Uwlī al-Nuhā" by al-Ruḥaybānī (3/90). .
This is for the following reasons:
Firstly: Because the option prevents entitlement to receive as long as the option remains; since entitlement is based on ownership, and the option prevents it [492]-See: " Tabyīn al-Ḥaqāʾiq" by al-Zaylaʿī (4/136). .
Secondly: Because the option does not allow for immediate exchange, and exchange must not be delayed [493] - See: "Mawāhib al-Jaleel" by al-Hattāb (6/137) .
Thirdly: Because what requires immediate receipt does not allow for postponement, and the option poses a greater uncertainty than delay; as it obstructs ownership or its obligation, thus it is more rightful to be excluded [494] - See: "Fatḥ al-ʿAzīz"  by al-Rāfiʿī (8/314) .
Fourthly: Because the purpose of considering receipt is for the parties to separate, maintaining no relation between them to avoid usury, or selling debts for debts; if we established the option, the relation would remain between them after separation [495] - See the previous reference. .
The Second Opinion: The option of condition in exchange is valid, which is the choice of Ibn Taymiyyah [496] -Ibn Taymiyyah said: "The option of stipulation is established in all contracts, even if the period is prolonged." See: "al-Fatāwā al-Kubrā" (5/390). , Al-Sa'di [497] - Al-Saʿdī said: "It is correct that the option of stipulation is established in leasing absolutely, as well as in exchange, salam, guarantee, and surety." See: "al-Mukhta-rāt al-Jalīyah" (p. 76). , and Ibn Uthaymeen [498] -Ibn ʿUthaymīn said: "The stipulation of an option contradicts this; therefore, the option of stipulation is not valid for what one has taken before separation as a condition for its validity. However, it is correct that it is established in exchange. We say: 'You may take possession before separation, and it remains with you according to what you have stipulated; either you proceed =with the sale or you cancel it,' due to the general statement of the Prophet (peace be upon him): 'Muslims are bound by their conditions.'" See: "al-Sharḥ al-Mumtiʿ" (8/281). .
The Evidences:
Firstly: From the Sunnah. The Messenger of Allah (صلى الله عليه وسلم) said: "Muslims are bound by their conditions. [499] -Al-Bukhārī reported it as a suspended narration with certainty before Hadith (2274). Al-Dārimī reported it in a connected form (3594), as did al-Hākim (2309), from the narration of Abū Hurayrah (رضي الله عنه). Ibn Ḥibbān authenticated it in his "Ṣaḥīḥ" (5091), and ʿAbd al-Ḥaqq al-Ishbīlī verified its chain in "Al-Aḥkām al-Ṣughra" (718). Al-Nawawī stated in "Al-Majmūʿ" (9/367) that its chain is good or authentic. Ibn al-Mulaqqin graded its chain as good in "Khilāṣat al-Badr al-Munīr" (2/69), and Ibn Kathīr affirmed it in "Irshād al-Fuqahāʾ" (2/54). Al-Albānī classified it as good and authentic in "Ṣaḥīḥ Sunan Abī Dāwūd" (3594).   "
Secondly: From the Athar (Saying of the Companions or their students).
Umar ibn al-Khattab رضي الله عنه said: "Indeed, the segments of rights are based on conditions, and you have what you stipulate. [500] -Al-Bukhari narrated it as a suspended report in a decisive form before Hadith (5151), and he also narrated it in a decisive form before Hadith (2721). It was narrated in a connected form by Ibn Abi Shaybah (16706) and by Sa'id ibn Mansur in his "Sunan" (663).    Al-Albani authenticated its chain of narration according to the conditions of the two Shaykhs in "Irwa' al-Ghalil" (6/304), and he confirmed it (1891) with the wording: "The portions of rights are according to the conditions. "
Thirdly: The fact that receiving is a condition for the validity of exchange does not prevent the establishment of the option; receiving occurs, and if it is not annulled, the objective is achieved, and if it is annulled, each returns what he has paid [501] -See: "Al-Makhtarat al-Jaliya" by Al-Sa'di (p. 76). .