Jesus' command doesn't apply today.
If Jesus' command to "freely give" (Matt 10:8) doesn't still apply today, and was just for his disciples, then it would be permissable to ask for money before telling someone about Jesus. Since that is obviously not permissable, the question is not whether we must "freely give" but what we must freely give.
Everyone can agree the basic message of the gospel must be freely given, but Jesus didn't just have the basic message in mind when he told his disciples to "freely give". While they were to freely proclaim "the kingdom of heaven" they were to also freely "Cure the sick, raise the dead, make the lepers clean, drive out demons".
Paul also ensured his own ministry was always free (1 Cor 9:18).
The reason Jesus told us to "freely give" was because we have "freely received" God's grace in our own lives. That has not changed for us, and so neither has Jesus' command.
The freely giving ethic
How Jesus' command to freely give works with Paul's teaching on financial support.
The scope of Jesus' command
Interpreting it too broadly or too narrowly misunderstands Jesus' expectation for ministry.