فَإِنَّ خَيْرَكُمْ أَحْسَنُكُمْ قَضَاءً
“For the best among you are those who repay best.”
إِنَّ adds emphasis. The attached كُمْ refers to the addressed group. أَحْسَنُكُمْ expresses comparison, while قَضَاءً specifies the area of excellence: settling an obligation.
لَا نَجِدُ إِلَّا أَفْضَلَ مِنْ سِنِّهِ
“We only find one better in age than his.”
لَا ... إِلَّا restricts the statement through negation and exception. This account does not prescribe an extra payment in all debts; it describes generosity in this settlement.