﴿ وَٱلَّذِينَ ءَامَنُوا۟ وَعَمِلُوا۟ ٱلصَّٰلِحَٰتِ لَنُكَفِّرَنَّ عَنْهُمْ سَيِّـَٔاتِهِمْ وَلَنَجْزِيَنَّهُمْ أَحْسَنَ ٱلَّذِى كَانُوا۟ يَعْمَلُونَ ﴾

“And as for those who attain to faith and do righteous deeds, We shall most certainly efface their [previous] bad deeds, and shall most certainly reward them in accordance with the best that they ever did.”

And those who believe and perform righteous deeds We will surely absolve them of their misdeeds by their performance of righteous deeds and We will indeed requite them with the best of what they used to do namely their righteous deeds it ahsana is in the accusative because the operator of the oblique bi has been omitted.