The author may Allāh have mercy upon him said Allāh does whatever He wills and this means that Allāh changes His creations in accordance to His Eternal Will which is not subject to change. Therefore whoever fulfills all the obligations and refrains from all the sin then that is because Allāh willed for that person success. Whoever disobeys and sins then this person has not been granted success by Allāh. This is the meaning of
“لا حول ولا قوة إلا بالله”
no one can evade sinning except with Allāh’s protection and no one has the ability to obey except with Allāh’s help.
The slave is not completely devoid of freewill and power but all of that is created by Allāh the one who created the self of that slave and so by priority Allāh is the creator of that slaves actions also.
The slaves are under Allāh’s will. Just as Allāh revealed the verse in Surah At-takweer that we don’t will except if Allāh the Lord of the universe wills.
The author may Allāh have mercy upon him said: “He is attributed with complete power over whatever He wills.” This means that Allāh The Exalted is attributed with complete power by which He makes things exist. He makes those things exist after they were non-existent. Therefore nothing renders him incapable and He does not need anything. Allāh revealed one of His attributes in Surah Al-Mā’idah in the verse that means And He has complete power over all things. Allāh’s power is not subject to defect, weakness or inability, instead Allāh’s power is complete just as it is revealed in the verse
إن الله هو الرزاق ذو القوة المتين
which means Indeed Allāh is the one who makes the sustenance reach the creations and He is attributed with complete power.