(And of the people is he who sells himself, seeking means to the approval of Allah. And Allah is kind to [His] servants)

The state of the believer in this world resembles the mukātab slave —and we are all slaves of Allah— a slave whose master has made a contract with him for a set sum of money; whenever the slave pays this money to his master, he frees him and he becomes free.

And the believer buys himself and frees himself from doubts, whims and desires, so that no debt of even an atom of whim remains on him but that he frees himself from it. And the more he increases in the bondage of servitude to Allah the Exalted, the more he is freed from the bondage of others: the world and created beings.

And in proportion to the perfection of servitude comes the beauty and sweetness of freedom.. That is the true freedom, the freedom of the spirit, not of wealth, body and self. That freedom in which a person truly and genuinely chooses what he loves and what he hates; a freedom with which he cares nothing for people's blame or praise; a freedom that began from the Truth and ended in Him, far removed from creation; a freedom stripped of the garment of fame, praise, blame, hatred and love — something above. From on high, how could it come down so as to be grasped by lower minds, those who clung to the world and whose souls became attached to it?

"So when he abandons disbelief and wickedness and proceeds to faith and obedience, it is as though he has bought himself from the punishment and the Fire. So the state of the believer becomes like the mukātab, who pays a counted number of dirhams and buys himself with them; likewise the believer spends a counted number of breaths and buys himself with them forever. But the mukātab is a slave as long as any dirhams remain owed by him, and likewise the one who is held accountable is not saved from the bondage of servitude as long as he has a single breath left in this world" al-Razi

He closed it with kindness, to affirm that whoever has these qualities (selling himself seeking the approval of Allah) attains Allah's good things and His mercies in this world and the Hereafter

"So kindness is a metonymy for what it entails, namely the bestowal of good things, like mercy

And the apparent meaning is that the definite article in His saying al-ʿibād (“the servants”) denotes comprehensiveness, because Allah is kind to all His servants, and they vary in it: among them are those whom Allah's kindness reaches in this world and in the Hereafter, with a variation in both that Allah's knowledge and wisdom require" Ibn Ashur

— Shoroog Almehmadi