زَاءً مِّن رَّبِّكَ عَطَاءً حِسَابًا (this being a reward from their Lord, a suffering grant...78:36). The blessings of Paradise described above is the reward of the believers and is an abundant grant from their Lord. Here two things are mentioned. First, these blessings are a reward of their deeds. Secondly, they are a Divine grant. Apparently, they seem to be contradictory, because reward means receiving something in lieu of doing some service, whereas grant is given gratis. The Qur'an has combined the two words in order to indicate that the blessings of Paradise are reward for the inmates of Paradise only apparently, and in reality they are a Divine grant given to them gratis, because human deeds cannot be the reward of even those blessings which were given to him in the world, let alone the blessings of the Hereafter which depend entirely on Divine grace as is confirmed by the Tradition of the Holy Prophet ﷺ : "No one can enter Paradise merely by virtue of his deeds, unless Allah shows His grace." The Companions asked: "What about you, Messenger of Allah ﷺ . "He replied: "Nor will I enter Paradise merely by virtue of my action."
The word hisaban has two meanings: [ 1] a grant [ that is ] sufficient [ and ] abundant. This meaning is adapted from the following idiom: اَحسبتُ فُلاناً ای اَعطَیتُہ، ما یَکفِیہِ حَتَٰی قَالَ حَسبِی 'ahsabtu means that I gave him so much so that it was sufficient for him until he yelled out "enough, this is too much for me"; and [ 2] the second meaning 'balancing of account' and 'comparing'. Sayyidna Mujahid ؓ says that in this context the verse purports to say that the Divine grant will be given gratis to the inmates of Paradise. The grant will be in keeping with the degree of sincerity as reported in authentic Traditions. The deeds of the noble Companions are greater than the deeds of the rest of the Ummah. If a Companion were to spend in the way of Allah one mudd = [ 815, 39 grams ] and a non-Companion were to spend to the equivalent of Mount Uhud, the Companion's one mudd will weigh heavier than the mountain. And Allah knows best!