It is important to remember that Christianisation was not a universal phenomenon in the antebellum South; efforts to bring slaves into the Christian fold begin on a meaningful scale only quite late in slavery's history, and the extent to which they were successful is highly debatable. For some context on this, I wrote this post about a month ago that you might interesting.

Something to keep in mind is that slaves were, by and large, illiterate. Plantation owners had little interest in teaching the majority of their slaves to read and write; agricultural work, after-all, did not require reading or writing. State laws in this period ranged from imposing limits on slave literacy to making it outright illegal.

In this context, pro-slavery preachers had quite a lot of freedom in what they could preach; it is not difficult to be selective in how you quote the Bible if your audience cannot pick up and copy and read it for themselves. As the slave narrative of Henry Bibb, published in 1849, notes:

This is where they have no Sabbath Schools; no one to read the Bible to them; no one to preach the gospel who is competent to expound the Scriptures, except slaveholders. And the slaves, with but few exceptions, have no confidence at all in their preaching because they preach a pro-slavery doctrine

Another narrative written by John Dixon Long, this one in 1857, tells us that:

They hear ministers denouncing them for stealing the white man's grain; but, as they never hear the white man denounced for holding them in bondage, pocketing their wages, or for selling their wives and children to the brutal traders of the far South, they naturally suspect the Gospel to be a cheat, and believe the preachers and the slaveholder to be in a conspiracy against them. In vain do we tell them the justice of the Father, of the precious promises in the Word of Life. Unbelief springs up; they doubt our sincerity, and say: "We poor slaves can't read, and how do we know what the Bible says!" Bitter experience has taught me some of these objections while laboring among them.

For the preachers on plantations then, seeking to advance a pro-slavery agenda, it would normally be quite straightforward for them to simply omit passages in the Bible that were not favourable to their cause, or distort their truth. Assuming that none of the slaves would be able to read the Bible if they wanted to, they had little fear of being undermined in this way.

That is not to say, of course, that there were not slaves who could read the Bible and found exception to the selective preaching of the planter class and their agents - there certainly were. Similarly, some white preachers and clergy did feel the need to address elements of scripture that condemned slavery, though this was to address challenges from the abolitionist community rather than from slaves themselves. On the subject of Moses, we can learn a great deal from an essay published in 1850 by Thornton Stringfellow, A Brief Examination of Scripture Testimony on the Institution of Slavery.

Stringfellow, a Baptist preacher from Virginia, discusses Moses quite a few times in his essay. He says, for example, that:

on the night the Israelites left Egypt, which was before the law was given, Moses, in designating the qualifications necessary for the passover, uses this language, Exod. xii. 44, 45: "Every man's servant that is bought for money, when thou hast circumcised him, then shall he eat thereof. A foreigner and an hired servant shall not eat thereof." This language carries to the human mind, with irresistible force, the idea of two distinct states--one a state of freedom, the other a state of bondage: in one of which, a person is serving with his consent for wages; in the other of which, a person is serving without his consent, according to his master's pleasure.

Later in the article, he goes on to note that:

the opinion has been announced also of late, that slavery among the Jews was felt to be an evil, and, by degrees, that they abolished it. To ascertain the correctness of this opinion, let the following considerations be weighed: After centuries of cruel national bondage practised upon Abraham's seed in Egypt, they were brought in godly contrition to pour out, "the effectual fervent prayer" of a righteous people, to the Almighty for mercy, and were answered by a covenant God, who sent Moses to deliver them from their bondage--but let it be remembered, that when this deliverance from bondage to the nation of Egypt was vouchsafed to them, they were extensive domestic slave-owners. God had not by his providential dealings, nor in any other way, shown them the sin of domestic slavery--for they held on to their slaves, and brought them out as their property into the wilderness. And it is worthy of further remark, that the Lord, before they left Egypt, recognized these slaves as property, which they had bought with their money, and that he secured to these slaves privileges above hired servants, simply because they were slaves. Exod. xii. 44, 45. And let it be noticed further, that the first law passed by the Almighty after proclaiming the ten commandments or moral constitution of the nation, was a law to regulate property rights in hereditary slaves, and to regulate property rights in Jewish hired servants for a term of years.

If you want to read the whole thing, you can find a searchable version of the text online here, in which Stringfellow comments on a number of aspects of the Moses story to construct his scriptural defence for slavery.