First the surah you sked about then the explanation that is very similar to my own understanding of this verse.
Verse:
[9:111] Surely Allah has purchased of the believers their lives and their belongings and in return has promised that they shall have Paradise.106 They fight in the Way of Allah, and slay and are slain. Such is the promise He has made incumbent upon Himself in the Torah, and the Gospel, and the Qur'an.107 Who is more faithful to his promise than Allah? Rejoice, then, in the bargain you have made with Him. That indeed is the mighty triumph.
Explenation:
106. In this verse that aspect of the Islamic faith which determines the nature of the relationship between Allah and His servants has been called a transaction. This means that faith is not merely a metaphysical conception but is, in fact, a contract by which the servant sells his life and possessions to Allah and in return for this accepts His promise that He would give him the Garden in the life after death. In order to comprehend the full implications of this transaction, let us first understand its nature.
We should note it well at the outset that, in reality, this transaction is not in regard to the actual selling of the life and possessions of the servant to Allah in the literal sense, for Allah is in fact the real Owner of man’s life and possessions. Allah alone has the right of ownership because He is the Creator of man and of everything he possesses and uses. Therefore there is no question at all of selling and buying in the worldly sense; for man possesses nothing of his own to sell, and Allah has no need to buy anything because everything already belongs to Him. However, there is one thing which has entirely been entrusted to man by Allah, that is, the freedom of will and the freedom of choice, and the transaction concerns that thing.
Of course, it is true that this freedom does not make any change in the real position of man with regard to the right of ownership to his own life and his possessions. They belong to Allah Who has delegated to him only the authority to use or abuse these things as he wills, without any coercion or compulsion from Him. This means that man has been given the freedom to acknowledge or not to acknowledge that Allah is the owner of his life and property. The transaction mentioned in Ayat 111 is concerning the voluntary surrender of this freedom to Allah’s will. In other words, Allah wills to test man whether he acknowledges the ownership of Allah over his life and property, in spite of that freedom, and considers himself to be their trustee only, or behaves as if he were their owner and so could do whatever he liked with them.
Thus, the terms of this transaction from Allah’s side are these: “If you voluntarily, and not by compulsion or coercion, agree to acknowledge that your life, your property and everything in this world, which in fact belong to me, are mine: And if you consider yourself only as their trustees; And if you voluntarily surrender the freedom I have given you to behave; And if you, in a dishonest way do not intend to become their master and owner; Then, I will give you in return, Gardens in the eternal life of the next world”. The one who makes this bargain with Allah is a believer, for faith is in fact the other name for making this bargain. On the other hand, the one who refuses to make this bargain, or after making it adopts the attitude of the one who has not made the bargain, is a kafir. For, technically, kufr is the term applied to the refusal to make this bargain.