وَمَنْ شُرِطَ لَهُ الْخِيَارُ فَلَهُ أَنْ يَفْسَخَ فِي مُدَّةِ الْخِيَارِ وَلَهُ أَنْ يُجِيزَهُ
Translation: The party given the option may rescind within the option period or affirm the sale.
Faskh means rescission; ijazah means affirming the sale as operative and binding.
The option is a real contractual power to choose within its prescribed period, not merely a chance to seek advice.
Example: the buyer has two days. Expressly confirming on day one is affirmation; declaring cancellation is an attempted rescission, subject to the additional rule in the next clause.