All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad  is His slave and Messenger.
  is His slave and Messenger.
If this person has clearly stated to you that he does not want any payment for his work and that he is offering it voluntarily, then he has no right to claim anything from you. However, we advise you to give him something as a kind gesture and appreciation.
The Prophet, sallallaahu 'alayhi wa sallam, encouraged rewarding those who do good to us, as he said: “Whoever does good to you, reward them for their goodness.” [Abu Dawood and others.]
Allah knows best.