خِيَارُ الشَّرْطِ جَائِزٌ فِي الْبَيْعِ لِلْبَائِعِ وَالْمُشْتَرِي، وَلَهُمَا الْخِيَارُ ثَلَاثَةَ أَيَّامٍ فَمَا دُونَهَا
Translation: A contractual option is valid for the seller and buyer; the option may be stipulated for three days or less.
The phrase lil-ba'i wa-l-mushtari establishes that the option is not restricted to a buyer. A seller can also reserve the option, and both may stipulate it.
Example: a book is sold for Rs 2,000 with an express two-day period for the seller to affirm or rescind. That illustrates the basic permission.