Let this be = This is. Psa_109:16 is a return to the subject of verses: Psa_109:1-5, and by the same speaker of verses: Psa_109:1-5.
reward = work. Not the same word as in Psa_109:5.
from the LORD = from Jehovah. It was He Who permitted it. Compare Psa_109:27, "This is Thy hand; Thou, LORD, hast done it". See Psa_22:15; Psa_38:2, Psa_38:3; Psa_39:9, Psa_39:10. Compare 2Sa_16:11, "the LORD hath bidden him".
speak evil. See verses: Psa_109:6-19 for the evil spoken.