I have no way of knowing, but Jesus is quoted directly throughout both of those Gospels.
There is a theory that a document, known as the Q document, contained the teachings of Jesus and was used by two or three of the Gospel writers.
I think I’ve also heard it suggested Matthew and/or Luke depended on the Gospel of Mark for some of its information.
Elijah,
The point of my question "Where did Mark and Luke get their quotes" was to direct you to the fact that the Holy Spirit is guiding the work of these men who are teaching the Gospel, whether orally or by written word. Naturally, Mark and Luke did not witness to Jesus in the flesh directly nor heard Jesus' words. They heard them from Peter and James and John and others. The words of Paul in the Scriptures come from the same source.
It is true, there is an appearance of development in the understanding of the Gospel - we can see that when comparing Galatians to Ephesians. However, we believe that the SAME SPIRIT is behind the inspired words written by either Paul or by Mark. Paul's words also originate from God.
As to your "contradiction", one must make some lattitude for issues in grammar and intent. Was Paul making an absolute statement that "all is fulfilled by one commmand"? Do we speak like that, not intending to use absolutes literally, but doing so for emphasis?
For example, if I say "The whole city was at the party", do I mean this in the literal sense, that every single human being within the city was present? When the Bible says we are saved by faith, does the rule out other places where the Bible says we are saved by hope, works, baptism or repentance? Or can it be that the author did not intend on making an absolute statement?
This is why it is important to read all the Scriptures and be more careful in assuming that Mark is more worthy of reading than James or Peter just because Mark quotes Jesus. (the later two knew Jesus, Mark did not...)
Regards