Allah says: do not consume one another's wealth unjustly, but only through trade by mutual consent (4:29). A sale needs the agreement of both parties, a lawful thing that is owned by the seller, a known price and a known item. The Prophet ﷺ said that the two parties of a sale have the option as long as they have not separated.
1The conditions
Mutual consent of both parties, free of force and cheating.
The seller owns the item and is able to deliver it. The Prophet ﷺ said: do not sell what is not with you.
The item and the price are known, and the item is lawful and useful.
The terms are clear, so that there is no gharar (excessive uncertainty).
2Options and defects
The option of the session (khiyar al-majlis): both parties can cancel as long as they are together, in the hadith. The Hanafi and Maliki schools do not take this literally and say the contract is binding on the agreement.
The option of defect: a buyer who finds a hidden defect may return the item or keep it with compensation.
Honest description: the seller must state faults he knows of. Hiding them is cheating.
3Conditions in contracts
Muslims are bound by their conditions, except a condition that makes the lawful unlawful or the unlawful lawful (Abu Dawud, graded hasan sahih by many).
There is to be neither harm nor reciprocating harm (Ibn Majah). This is a general rule of fiqh.
Where the schools differ
The four Sunni schools agree on the main ruling above and differ in these points. No school is preferred here.
The option of the session. the Shafi'i and Hanbali schools apply the hadith literally, and the Hanafi and Maliki schools hold that the contract is binding as soon as the offer and acceptance are made.
Hai orang-orang yang beriman, janganlah kamu saling memakan harta sesamamu dengan jalan yang batil, kecuali dengan jalan perniagaan yang berlaku dengan suka sama-suka di antara kamu. Dan janganlah kamu membunuh dirimu; sesungguhnya Allah adalah Maha Penyayang kepadamu.
Hai orang-orang yang beriman, apabila kamu bermu'amalah tidak secara tunai untuk waktu yang ditentukan, hendaklah kamu menuliskannya. Dan hendaklah seorang penulis di antara kamu menuliskannya dengan benar. Dan janganlah penulis enggan menuliskannya sebagaimana Allah mengajarkannya, meka hendaklah ia menulis, dan hendaklah orang yang berhutang itu mengimlakkan (apa yang akan ditulis itu), dan hendaklah ia bertakwa kepada Allah Tuhannya, dan janganlah ia mengurangi sedikitpun daripada hutangnya. Jika yang berhutang itu orang yang lemah akalnya atau lemah (keadaannya) atau dia sendiri tidak mampu mengimlakkan, maka hendaklah walinya mengimlakkan dengan jujur. Dan persaksikanlah dengan dua orang saksi dari orang-orang lelaki (di antaramu). Jika tak ada dua oang lelaki, maka (boleh) seorang lelaki dan dua orang perempuan dari saksi-saksi yang kamu ridhai, supaya jika seorang lupa maka yang seorang mengingatkannya. Janganlah saksi-saksi itu enggan (memberi keterangan) apabila mereka dipanggil; dan janganlah kamu jemu menulis hutang itu, baik kecil maupun besar sampai batas waktu membayarnya. Yang demikian itu, lebih adil di sisi Allah dan lebih menguatkan persaksian dan lebih dekat kepada tidak (menimbulkan) keraguanmu. (Tulislah mu'amalahmu itu), kecuali jika mu'amalah itu perdagangan tunai yang kamu jalankan di antara kamu, maka tidak ada dosa bagi kamu, (jika) kamu tidak menulisnya. Dan persaksikanlah apabila kamu berjual beli; dan janganlah penulis dan saksi saling sulit menyulitkan. Jika kamu lakukan (yang demikian), maka sesungguhnya hal itu adalah suatu kefasikan pada dirimu. Dan bertakwalah kepada Allah; Allah mengajarmu; dan Allah Maha Mengetahui segala sesuatu.
From the Sunnah
Sahih al-Bukhari 2107Authentic collection
The point of the hadith: The buyer and the seller have the option to cancel or to confirm as long as they have not separated.
Read the Arabic text
حَدَّثَنَا صَدَقَةُ، أَخْبَرَنَا عَبْدُ الْوَهَّابِ، قَالَ سَمِعْتُ يَحْيَى، قَالَ سَمِعْتُ نَافِعًا، عَنِ ابْنِ عُمَرَ، رضى الله عنهما عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ " إِنَّ الْمُتَبَايِعَيْنِ بِالْخِيَارِ فِي بَيْعِهِمَا، مَا لَمْ يَتَفَرَّقَا، أَوْ يَكُونُ الْبَيْعُ خِيَارًا ". قَالَ نَافِعٌ وَكَانَ ابْنُ عُمَرَ إِذَا اشْتَرَى شَيْئًا يُعْجِبُهُ فَارَقَ صَاحِبَهُ.
This is general information, not a fatwa. For your own situation, ask a qualified scholar.
Fiqh questions
Conditions of a valid sale
Allah says: do not consume one another's wealth unjustly, but only through trade by mutual consent (4:29). A sale needs the agreement of both parties, a lawful thing that is owned by the seller, a known price and a known item. The Prophet ﷺ said that the two parties of a sale have the option as long as they have not separated.
Is this a fatwa?
No. It is a summary of well-known rulings with their evidence. For your own case, ask a qualified scholar.
What if my school differs from this answer?
Follow the view of the school you follow and the scholar who teaches you. The page lists the differences of the four schools.