22 Moses was educated in all the learning of the Egyptians, and he was a man of power in words and deeds.
NASBu  Acts 7:22

10 Then Moses said to the LORD, ‘Please, Lord, I have never been eloquent, neither recently nor in time past, nor since You have spoken to Your servant; for I am slow of speech and slow of tongue.’
NASBu  Exodus 4:10

14 Then the anger of the LORD burned against Moses, and He said, "Is there not your brother Aaron the Levite? I know that he speaks fluently. And moreover, behold, he is coming out to meet you; when he sees you, he will be glad in his heart.
15 You are to speak to him and put the words in his mouth; and I, even I,
will be with your mouth and his mouth, and I will teach you what you are to do.
16 Moreover, he shall speak for you to the people; and he will be as a mouth for you and you will be as God to him.’

NASBu  Exodus 4:14-16

12 But Moses spoke before the LORD, saying, ‘Behold, the sons of Israel have not listened to me; how then will Pharaoh listen to me, for I am unskilled in speech?’
NASBu  Exodus 6:12

30 But Moses said before the LORD, ‘Behold, I am unskilled in speech; how then will Pharaoh listen to me?’
NASBu  Exodus 6:13

SAB Contradiction 1240 (by book)

Bible Contradiction?

We have in the Bible three confessions of Moses that he is not an eloquent speaker. We also have the confirmation of God who gave Moses his brother Aaron to speak for him on serious occasions. And now we hear Stephen say that Moses was ‘a man of power in words’ or as the KJV expresses it: ‘mighty in words’. Did Stephen better know than Moses and even God Almighty?

Not identical

If we look at the difference in wording, the solution is quickly found.
The phrases ‘power in words’, ‘mighty in words’ are not identical with ‘being an eloquent speaker’.

1. Power in words, mighty in words: these phrases are dealing with authority in speech. That is to obtain by knowledge or social position, or both. It is not necessary in this case to use fine words, speaking fluently or even poetically, to strike the right note or whatever.   
2. Eloquent speaking: it is certainly possible to train one’s speaking to become a successful speaker. And of course during the training it will be stressed that a good speaker is also an informed speaker; but without the talent to grip the audience one will never be a fascinating speaker.

A missing talent

Moses complained that he lacked the gift of speaking eloquently. He knew that he lacked the talent to convince people in a simple and natural way. But as the Prince of Egypt, when he spoke everyone immediately became silent. He was educated in all Egyptian knowledge; he knew what he was talking about. In particular, his social position forced the people to not be inattentive when he opened his mouth.

Need for a helper

Later on when he had lost his position as Prince of Egypt, it was impossible for him to fall back on speaking with power because of having a high status. In the confrontation with the Pharaoh, and in the convincing of his own people, he was in terrible need of a helper who could speak for him. So God gave him his brother, Aaron, to do so.

Stephen was not confused

Both are true. Moses, being a Prince, was mighty in words, without being eloquent.
These aspects do not contradict, but they are complementary. Stephen was right. His hearers, who were intelligent Jews, didn’t have any problem with Stephen’s description of Moses’ abilities. Neither do we: no Bible Contradiction at all.

No Bible Contradiction