There are several major differences between the U.S. and Confederate constitutions in the area concerning slavery.Whereas the original U.S. Constitution did not use the word slavery or the term "Negro Slaves", but "Person[s] held to Service or Labor" which included whites in indentured servitude, the Confederate Constitution addresses the legality of slavery directly and by name.
Though Article I Section 9(1) of both constitutions are quite similar in banning the importation of slaves from foreign nations the Confederate Constitution permits the C.S. to import slaves from the United States and specifies the "African race" as the subject. The importation of slaves into the United States, including the South, had already been illegal since 1808.
Article I Section 9(1)
The importation of negroes of the African race from any foreign country, other than the slaveholding States or Territories of the United States of America, is hereby forbidden; and Congress is required to pass such laws as shall effectually prevent the same.
While the U.S. Constitution reads
The Migration or Importation of such Persons as any of the States now existing shall think proper to admit, shall not be prohibited by the Congress prior to the Year one thousand eight hundred and eight, but a tax or duty may be imposed on such Importation, not exceeding ten dollars for each Person.
The Confederate Constitution then adds a clause that the C.S. Congress has the power to prohibit the importation of slaves from any state that is a non-Confederate state.
Article I Section 9(2)
Congress shall also have power to prohibit the introduction of slaves from any State not a member of, or Territory not belonging to, this Confederacy.
While the U.S. Constitution has a clause that states "No bill of attainder or ex post facto law shall be passed" the Confederate Constitution adds a phrase to explicitly protect slavery. According to historian William C. Davis, the fact that the Confederate constitution explicitly protected slave ownership, "should hardly have surprised anyone."
Article I Section 9(4)
No bill of attainder, ex post facto law, or law denying or impairing the right of property in negro slaves shall be passed.
The U.S. Constitution states in Article IV Section 2 that "The Citizens of each State shall be entitled to all Privileges and Immunities of Citizens in the several States." The Confederate Constitution adds that a state government cannot prohibit the rights of a slave owner traveling or visiting from a different state with his or her slaves.
Article IV Section 2(1)
The citizens of each State shall be entitled to all the privileges and immunities of citizens in the several States; and shall have the right of transit and sojourn in any State of this Confederacy, with their slaves and other property; and the right of property in said slaves shall not be thereby impaired.
The Confederate Constitution added a clause about the question of slavery in the territories (the key Constitutional debate of the 1860 election) by explicitly stating that slavery is legally protected in the territories.
Article IV Section 3(3)
The Confederate States may acquire new territory; and Congress shall have power to legislate and provide governments for the inhabitants of all territory belonging to the Confederate States, lying without the limits of the several states; and may permit them, at such times, and in such manner as it may by law provide, to form states to be admitted into the Confederacy. In all such territory, the institution of negro slavery as it now exists in the Confederate States, shall be recognized and protected by Congress, and by the territorial government: and the inhabitants of the several Confederate States and Territories, shall have the right to take to such territory any slaves lawfully held by them in any of the states or territories of the Confederate states.