You can never be certain? Really? Of course you don’t see it as a bribe, but your uncertainty speaks volumes. The reasoning is “I give to God and He has to bless me because I have ‘done my part’”. That is at least the Mormon angle and I see it in Christian churches far too much.
Perhaps I am a stickler on the ‘tithing’ issue because of my Mormon background, but I don’t believe 10% is biblical.
IT IS ALL GOD’S MONEY. Showing gratitude is proper motivation, not expecting blessings. God will bless you as He will, regardless of what you do or no not give - He is God.
I give as I am led, I do not give 10% and I give where I am led.
Just be careful that you don’t assume you are blessed only because you tithe, brother.
God speed.
...and since the BIBLE says that NOTHING can be added to or taken away from a duly agreed upon contract...
Unless you are JEWISH; then you are probably right.