“Whoever is generous to the poor lends to the LORD, and He will repay him for his deed.” Proverbs 19:17
The LORD promises that when we give to those who don't have or are in need, He will repay us. He will give back to us.
It is even asserted in the New Testament that if we see a brother or sister in need and we don't give to them, we don't actually possess the love of GOD within us. It is that serious.
Another somber warning and promise is found in Malachi.
“‘Return to Me, and I will return to you, ’says the LORD Almighty. But you ask, ‘How are we to return?’ Will a mere mortal rob God? Yet you rob me. But you ask, ‘How are we robbing you?’ In tithes and offerings. You are under a curse—your whole nation—because you are robbing Me.” Malachi 3:7–9
By giving the first portion of our income to the Lord, as we are blessed, and by removing it from our possession (so that we don't use it or forget to hand it over to Him), we are practicing what God exhorts us to in Malachi above -- we are "returning to Him". He, in turn, will "return to us" not just material blessings, but spiritual ones, as well.
Sadly, the opposite holds true, too. If we fail to give to God what is His anyway, then we are breaking covenant with him and robbing him of his hallowed portion of our blessings. The wisest man ever to live, Solomon wrote in Proverbs chapter 3, verse 9, that we should "honor the Lord with [our] wealth and with the first fruits of all [our] produce."
Again the prophet Malachi tells us that the LORD YHWH promises that if we are faithful and obedient in honoring Him, He will "return to us" by rebuking the devourer that prowls around to destroy us, physically, spiritually and financially.
"Then I will rebuke the devourer for you, so that it will not destroy the fruits of the ground; nor will your vine in the field cast its grapes, says the LORD of hosts.” Malachi 3:11
If we do not obey God, then 1) How can we really call him our Father?, and 2) How can we possibly expect Him to bless or prosper us financially?
One of the first and most important acts of obedience and honor we can show YHWH if we are indeed His people is to remove the firstfruits of our increase, the first tenth (or tithe0, what is know as the sacred or holy portion, so that we may give it to those who serve the Lord, as well as to the poor, orphans and widows.