إِذَنْ follows a complete statement and introduces its result or the listener's reaction.
قُلْتُ لِزَيْدٍ: هَذَا الْكِتَابُ مُفِيدٌ، فَقَالَ: إِذَنْ سَأَقْرَؤُهُ
I told Zayd, “This book is useful,” so he replied, “Then I shall read it.”
The useful book is the first statement; deciding to read it is the reaction. Here سَـ stands between إِذَنْ and the verb, so this example teaches meaning, not a visible final fatḥah.