«لَكِنْ» لِلِاسْتِدْرَاكِ بَعْدَ النَّفْيِ فَيَكُونُ مُوجِبُهُ إِثْبَاتَ مَا بَعْدَهُ
Lakin introduces a qualification after negation, with its positive effect directed at what follows.
In “not X, but Y,” the preceding negation remains in place, while Y is asserted. For example, “It is not Zayd, but Amr” denies one identification and asserts another.
Unlike a simple replacement reading of bal, lakin does not itself invalidate the preceding negation. The denial and subsequent affirmation must be interpreted together.