There is no contradiction. Paul had a complex view of the law that many authors have tried to get a handle on. Paul acknowledges the power of the law, but through the law died to the law (Gal. 2.19), and so is not under the law (Gal. 5.18). The law was fulfilled by Christ (Gal. 3.24). Jesus claims the same thing in Mt. 5.17-20. Where's the contradiction?

Does Paul's teaching contradict Matthew 5:17-20?

Romans 6:14
"For sin shall no longer be your master, because you are not under the law, but under grace."

The law has no glory at all in comparison with the New Covenant. (2 Corinthians 3:10)

Christians are dead to the law. (Galatians 2:19)

As the Gospel of Matthew is written after Paul, it seems that they contradict each other.