مَن جَاءَ بِالْحَسَنَةِ فَلَهُ خَيْرٌ‌ مِّنْهَا

"Whoever will come with the good deed will have (a reward) better than that" - 27:89

This description relates to the situation that will take place after the Doomsday and the account taking. Here the word hasanah is purported for the kalimah of لا إله إلا اللہ محمد رسول اللہ (as explained by Ibrahim) or it is an expression of sincerity (as interpreted by Qatadah). Some have interpreted this to include complete submission. The sense is that every good deed will be recompensed by a reward better than the deed; but it should be remembered that a person's righteous acts could be considered as righteous only when he fulfills its first condition of having faith in Allah Ta` ala. Then he would get an award better than commensurable to his good deeds. It means that he would be graced with eternal bounties of the Paradise, and would escape from all troubles and torments for good. Some have interpreted that the word "better" means here that the reward of one good deed would be awarded from ten times to seven hundred times more. (Mazhari)

وَهُم مِّن فَزَعٍ يَوْمَئِذٍ آمِنُونَ

and such people will be immune on that day from any panic -27:89.

Word Faza' is purported here for any big calamity or perplexity. Thus the meaning of the sentence is that, while living in this world, every pious person is scared of his fate in the Hereafter, and as a matter of fact, he should be scared in the light of Qur'anic verse إِنَّ عَذَابَ رَ‌بِّهِمْ غَيْرُ‌ مَأْمُونٍ (70:28). The torment of Allah is not something to be forgotten and set aside. It is for this reason that even the prophets, the companions, and saints were always frightened and daunted. But after the account taking on Doomsday those who would bring the hasanah (the good deed contemplated in this verse) with them would be free from all fear and grief, and will live forever in complete tranquility. (Only Allah knows best)