"Bring the whole tithe into the storehouse, so that there may be food in My house, and test Me now in this," says the LORD of hosts, "if I will not open for you the windows of heaven and pour out for you a blessing until it overflows."
It is impossible to out give God. If we are willing to trust Him, and give generously of our resources (which by the way, all belong to Him anyway), He will provide more than we need. Much more. God is faithful and generous. If He says He'll do it, He will.