1 Corinthians 7:22

For he that is called in the Lord, being a servant, is the Lord's freeman: likewise also he that is called, being free, is Christ's servant (KJV)

Then: How can someone be free and be a servant (slave) at the same time? How can someone be a servant (slave) and be free at the same time? Freed in Christ, Paul answers. He knows that the bondages of sin along with its guilt are the true destroyers of man's spirit and life. Being forgiven and freed from sin completely changes the believer's attitudes and purpose. In order to explain, Paul uses the first century social structure terms because that's the world he and the Corinthians inhabited. When Christ died he paid the price for our freedom, just as a price is paid for the servant's freedom. Paul says ignore the temporary condition on earth. The freedom and liberty of the soul demands utmost attention.

Now: Two issues regarding ultimate goals and social status cause problems in today's western world. Materialism is the first. N. T. Wright (p. 86) emphasizes that God uses the nobodies of the world, "the least of these," to confound the smart ones who can't get life's true priorities sorted out. The pursuit of possessions often tempts and side tracks believers. Satan laughs because so few live the life of Mother Teresa.
The second, racism, is more difficult to explain. Racism is a worldly attitude based on status. Western concepts of slavery differ from that of the first century institution, yet commentators look for Paul's understandings and principles. In the first century, being a servant could give high status just like being chosen to work for a Fortune 500 company does today. Older slaves could be released from service. In both eras, servitude did involve giving up the possibility of having personal possessions, choosing vocations, having free time, or receiving payment for work tendered.* Phillips (p.155-156) gives an explanation that sometimes owners would pay a temple god a fee in order to free a slave. From then on the freeman could never again be made to be a slave.
Today, the burden of racism hinders those seeking opportunities to own homes, choose vocations, have leisure time, and work for payment. From the opposite view, understanding of individuals trying to overcome generational attitudes of victimhood is lacking. Again Satan laughs because so few follow the footsteps of Martin Luther King. Believers need Paul's advice. Was he saying that slaves and/or victims of racism should always stay where they are? No. He knew believers' are required to pray and work for God's kingdom on earth. We are expected to focus on Christ's values and purposes, not those of the world. Liberty in Christ is an interim status. Having the mind of Christ is the goal. Check out the next verse.

Extra source: *Thomas Sowell in April 20, 2010 article "The Limits of Power," explains that many - such as Rome, US Old South, modern China - who used involuntary servitude plus whipping could not get good work from slaves doing technical, intellectual tasks unless they were also given money.
Exploring 1 Corinthians: an expository commentary by John Phillips. Grand Rapids MI: Kregel Publications, c2002. http://books.google.com/books?id=c4F0l2b1QnkC&printsec=frontcover&dq=1+Corinthians+and+Phillips&source=bl&ots=NVKHtw_lSu&sig=JuRG1-pmT2bK7O0KAZUDxClRUpo&hl=en&ei=CTXPS9nwMpTC8wS5mrHCDw&sa=X&oi=book_result&ct=result&resnum=4&ved=0CA8Q6AEwAw#