God gave Jesus in faith that we might one day give our lives to Him. The gift of his Son came before the blessing of our repentance and salvation. We give our first-fruits in much the same way.
Before we see the blessing of God, we give it in faith.
Giving the first-fruits of your income says to God "I recognize you first. I am putting You first in my life, and I trust You to take care of the rest."