Article1, Section 9

"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."

This was how they chose to deal with the problem of slavery. Many of the contributors found slavery abhorrent, but many were southerners, and many owned slaves themselves, notably among them Thomas Jefferson and Ben Franklin. They dealt with it in this way because they knew they'd never get the deal done if they tied the question of slavery to the acceptance of the Constitution.

Article 4, sect 2.

"(No Person held to Service or Labour in one State, under the Laws thereof, escaping into another, shall, in Consequence of any Law or Regulation therein, be discharged from such Service or Labour, But shall be delivered up on Claim of the Party to whom such Service or Labour may be due.) (This clause in parentheses is superseded by the 13th Amendment.)"

this made it illegal to assist a fugitive slave. Hence it was superseded by the 13th Amendment which as we both know, since
WE BOTH
have read the Constitution, Abolished Slavery.