The Fruit of any sustained increase in your income such as a raise, is given to God. This is considered the First Fruit. Unlike the tithe which is a 10th of your paycheck and is likely given weekly or monthly, the First Fruit is given once. Therefore, if I receive a $100 increase in my salary, the first $100 goes to God. In all subsequent paychecks, you keep the increase.
“Give God the first, and the best, and HE will bless the rest”.
All firsts belong to God! — “Honor the LORD with your possessions, and with the firstfruits of all your increase; so your barns will be filled with plenty, and your vats will overflow!” (Proverbs 3:9 – 10 NKJV).
When you come into the land which I give you and reap its harvest, then you shall bring a sheaf of the first fruits of your harvest to the priest” (Leviticus 23:10 NIV).