In this reading, the noun is accusative as the predicate of kāna (i.e. taku), linking back to the measure of “a speck”.

In this reading, the noun is nominative as the subject of kāna with the sense of “if it should exist/occur”.

Abū Jaʿfar al-Madanī

Ibn ʿĀmir al-Shāmī

إِنَّ ٱللَّهَ لَا يَظْلِمُ مِثْقَالَ ذَرَّةٍ ۖ وَإِن تَكُ حَسَنَةً يُضَـٰعِفْهَا وَيُؤْتِ مِن لَّدُنْهُ أَجْرًا عَظِيمًا