Message of 16:75 and 16:76
The significance of the examples cited

16:75

God cites an example: a slave who is owned and cannot do a thing, and one whom We have granted good provisions from which he gives secretly and publicly. Are they the same? Praise be to God! In fact, most of them do not know.
16:75

In this verse, God compares a righteous servant whom God blessed with provisions and who in turn was charitable, to a slave who cannot do a thing.
The words "Are they the same?" indicate that these two are not equal in the sight of God. The second man who gives secretly and publicly is better in the sight of God than the slave.

The question that arises here is how can a slave be condemned for what he is?
No one is a slave by choice, rather it is God who decrees all such matters:

He raised some of you above others in rank so as to test you through what He has given you.
6:165

An important indicator in the understanding of this verse is found in the words
"God cites an example". This phrase tells us that the descriptions in this verse are not to be taken literally. The verse cannot be speaking about a literal slave for the simple reason that being a slave is not a reason to earn God's condemnation. There must be another form of slavery that earns God's condemnation. Indeed, the Quran tells us about a different category of slaves who earn God's condemnation.

In the following verse we find the answer we are looking for:

The devil has secured a firm grip on them
, and so has caused them to forget the remembrance of God. Those are the party of the devil. Without a doubt, it is the devil's party who are the losers. 58:19

The fact that the devil has a
"firm grip" on those who are the "party of the devil" indicate that such category of people are indeed enslaved by him. They are the ones who followed in the footsteps of the devil despite God's warning (2:168). They are the ones who allowed the devil to lead them far astray (4:60) with his call for immorality and evil (24:21), and they are the ones who let the devil succeed in repelling them from the remembrance of God (5:91). The devil was able to achieve the above results by adorning such vile acts to his followers (16:63).
In electing to follow in the footsteps of the devil such people allow themselves to be totally captivated by the devil (6:71) and to allow him to have a firm grip on them (58:19). This is nothing other than being enslaved by the devil.
The words in 16:75 mark the difference between the righteous charitable believers and those who are enslaved by the devil due to their own choices. Indeed, they are not the same in the sight of God.

16:76

And God cites an example: two men, one of them is dumb and cannot do a thing and is a burden on his master. Wherever he sends him, he brings back nothing good. Is he the same as one who advocates justice and is on a straight path?
16:76

In this verse, God compares one who
"advocates justice and is on a straight path" to one who is dumb and cannot do anything right.
The same question arises: What is the fault of someone who is dumb by nature to be in any way condemned by God?

Once again, we are reminded that the words are not literal but symbolic.
God is the Creator of everything in the heavens and the earth including the human being. It is God who created some people with intelligence and others with simple minds and also some who are indeed dumb. As a result, it is not plausible that God would look down on any person who is created dumb without having any choice in how he is created. This compels us to search the Quran for a different category of people who are described as dumb.

When we search the Quran, we find one category of people whom God consistently calls dumb, not dumb by creation but by the choices they made:

Those who denied Our revelations are deaf and dumb
in utter darkness. Whomever God wills, He sends astray, and whomever He wills, He sets on a straight path. 6:39

Those who deny God's revelations are indeed deaf and dumb. Denying God's revelations was their own choice and not one that was enforced on them.
The words in 16:76 therefore provides a comparison between those who deliberately chose to deny God's revelations to those who are on the straight path and who advocate justice. Indeed, those two categories are not the same in the sight of God.