Firstly: Defining “form”
Linguistically: To “form” something is to mould it in an even form, shape it, and fashion it. To “form” poetry or speech is to compose and arrange it. This is the “form” (sawgh) of that means “of equivalent value”, and its “form” (sighah) is such-and-such means the shape in which it was constructed. [209] Lisan al-`Arab by Ibn Manzur (8/442) and Taj al-`Arus by Zabidi (22/533, 536).
Technically: That which indicates the consent of the two parties to a transaction, whether in word or deed. [210] Mawahib al-Jalil by Hattab (4/228). See also Al-Insaf by Mardawi (4/190) and Al-Bahr al-Ra’iq by Ibn Nujaym (5/278).

Secondly: The ruling of format (i.e., offer and acceptance)
The format is one of the pillars of a transaction, and that is agreed upon by the four jurisprudential schools of thought: [211] They do differ regarding the specific details of the contract forms – what is valid and what is not – but they unanimously agree that the formulation of the contract is one of the pillars of a [valid] trade. The Hanafis, [212] Al-Bahr al-Ra’iq by Ibn Nujaym (5/278). See also Bada’i` al-Sana’i` (5/133). Malikis, [213] Al-Taj wa al-Iklil by Mawwaq (4/228) and Mawahib al-Jalil by Hattab (6/13). Shafi`is, [214] Fath al-`Aziz by Rafi`i (8/99), Al-Majmu` by Nawawi (9/149), and Rawdat al-Talibin by Nawawi (3/338). and Hanbalis. [215] Sharh Muntaha al-Iradat by Bahuti (2/5) and Kashshaf al-Qina` by Bahuti (3/146).

Evidence from the Book:
The Statement of Allah Exalted, “O believers! Do not devour one another’s wealth illegally, but rather trade by mutual consent.” [216] al-Nisa’, 29.
Achieving consent is one of the pillars of a transaction, and the evidence of consent is the format of the contract (i.e., response and acceptance). [217] Mawahib al-Jalil by Hattab (6/13).