Regarding Khulʿ: Explain the saying (Will you return his garden to him)?

What you asked about is part of a hadith (definition) narrated by Al-Bukhari and others, on the authority of Ibn Abbas and others, concerning the wife of Thabit ibn Qays, who wished for khul' (divorce at the wife's instance) and disliked staying with him. The Prophet (peace be upon him) asked her: "Will you return his garden to him?" She said: "Yes." The Messenger of Allah (peace be upon him) said: "Accept the garden and divorce her with a single pronouncement." The "garden" here refers to the orchard he had given her as a mahr (dowry). Scholars have derived from this hadith the legitimacy of khul' between spouses.