The fact that Mark, in Mark 6, does not even purport to give Jesus' words but rather that the narrator expressly summarizes Jesus' instructions to his disciples when sending them out and that Matthew actually does give an appearance of direct quotation is some evidence that Matthew is not merely "editing Mark" at this point. The hypothesis of eyewitness testimony absolutely does make a difference to what possibilities are on the table. While it is not impossible that Matthew was merely putting into direct quotation what Mark puts in indirect quotation, we also need to get rid of rigid redaction-critical assumptions that, if an incident is both in Mark and Matthew, Matthew is merely getting his information from Mark. Again and again Matthew may well be adding information, based upon memory, that Mark did not have. In this case, a well-known Bible difficulty concerns the fact that Mark summarizes (in the voice of the narrator) that Jesus said not to take anything except a staff, whereas Matthew says not to "take," inter alia, a staff.
But as has been noted by old-style inerrantists for a very long time, the Greek word in Matthew is "acquire." Since Matthew may actually have been a disciple, he may actually have remembered that Jesus said not to acquire these items rather than that they were to discard a staff they already had. Luke, who may at this point indeed have been dependent upon both Mark and Matthew, combines the two (using the appearance of direct quotation) by using the general word "take" from Mark and listing a staff, as in Matthew, as one of the things that they were not to "take." But Matthew's more precise use of "acquire" can help us to understand Luke's approximate quotation at this point better as well.
This is not to say that Matthew's quotation is absolutely verbatim, word-for-word, as a tape recorder, either. But it is to say that his use of "acquire" is helpful and may well indicate what an eyewitness remembered more specifically that Jesus said, especially since Mark does not even give the appearance of quoting Jesus directly.
We must take more seriously the hypothesis of eyewitness testimony giving us additional insight into actual events. Again and again, critical scholars ignore this hypothesis, to the detriment of our understanding of Jesus' words and actions. It is overly restrictive to be constantly insisting to the laity that in any such case they must simply accept that Matthew and Luke "edited Mark," as though the hypothesis of additional witness testimony is simply off the table as a useful explanation of what we have. While it is certainly true that witnesses do moderately paraphrase what they have heard and witnessed, that is not all that they do. They also remember additional information. The word "acquire" in Matthew is part of what we observe. The possibility of separate witness testimony to Jesus' use of such a term is a perfectly plausible explanation.