1) Matthew 10:8b features Jesus saying, "You received without payment; give without payment" (NRSV). This part of verse 10:8 seems rather ambiguous because the only reference to what the disciples "received" and are thus to "give" freely is the power to heal in v. 1. Additionally, there is no subsequent stories in the Gospel where the disciples use or give these powers; only Jesus heals and casts out spirits. Initially, this sentence seems a little out of place and not explicitly related to the preceding or following verses.
2) a. Davies and Allison:
They argue that this verse does continue the "rhythm of the preceding couplets" in the Greek, but may be redactional in origin (170). The basic meaning could be simply that Jesus is enforcing the importance of giving to others what God has graciously given to them: the power to heal (171). Such commands for reciprocation are found in other places in Scripture, notably in 2 Kg. 5:15. Their reading of this passage reflects this need for reciprocation both in appreciation for God's gift and as a means to spread the gospel effectively.
This small verse, according to Luz, represents a framing of vv. 9-10 and initially appears to be in contention with v. 10b: "...for laborers deserve their food" (NRSV) (76). Luz adds that this verse ties the disciples' gift to Jesus' gift from God. He cleverly notes the authors use of the Greek word trofh, (food) in instead of misqo,j (wages/pay) which is found in one of Matthew's sources (76), basically to say that 10b references 8b, arguing that the laborers will receive food (spiritual? physical?), not a salary for what they do (76). Thus Luz reads this passage to say that the gift given to the disciples for the power to heal and perform miracles concerning the gospel should never accrue financial compensation or debts.
3) An initial reading of v. 8b may seem as if this verse does not fit well into Christ's instructions for the disciples in vv. 5-15, but looking closer, it seems as if Jesus may simply wish for the disciples to remember the great gift that has been given to them without a price. Likewise, the disciples' remembrance of this event from v. 1 should be enough to compel them to share this gift with those worthy few that they are sent to serve. The relationship to 10b may reveal the importance of never requesting or accepting any monetary payment, but rather more practical survival needs like food for their services and miracles. Perhaps the concern is if money is involved, it could compromise the genuineness of the gospel. The lack of evidence in this Gospel as a whole for the disciples doing this adds weight to the idea that it is odd that so much emphasis is placed here on the gift of healing and casting out that the disciples are given.