The Ashari school inherited a problem rather than invented one. If God creates everything, including what a person does, then blaming or rewarding that person looks unjust. If a person creates their own acts, then something in the world has been made by someone other than God. The Mutazila took the second horn and argued that human beings genuinely originate their acts, which is what makes the final reckoning fair. Abu al-Hasan al-Ashari, who had studied in that school under al-Jubbai before breaking with it, took neither.
His answer is kasb, usually translated as acquisition. God creates the act at the moment it occurs, and the human being acquires it, meaning that the act is genuinely theirs in the sense that matters for praise and blame without their having brought it into existence. The power exercised in the act is itself created by God alongside the act, so nothing is added to the world by a creature. What the person contributes is not creation but the fact that the act attaches to them and to nobody else.
Critics inside and outside the school have said for a thousand years that acquisition names the problem rather than solves it, since it is hard to say what acquiring adds if the power to act is created too. Later Ashari writers worked on exactly that objection, and al-Baqillani and al-Juwayni did not agree with each other about the answer. The doctrine has survived not because it satisfied everyone but because it holds together the two commitments the school would not surrender: that God is the only creator, and that a person is answerable.