"Adam slept with Eve his wife. She conceived and had Cain. She said, "I've gotten a man, with God's help!"
Then she had another baby, Abel. Abel was a herdsman and Cain a farmer. Time passed. Cain brought an offering to God from the produce of his farm. Abel also brought an offering, but from the firstborn animals of his herd, choice cuts of meat. God liked Abel and his offering, but Cain and his offering didn't get his approval. Cain lost his temper and went into a sulk."
Why did God not approve of Cain and his offering? Some have taught that Abel's sacrifice was brought early and quickly, a supposed "first-fruits" offering, a justification for teaching tithing for current day believers. In certain translations, with lots of stretching, this might be seen, but it's a stretch at best. Although as you can see from the translation above, time passed before either Cain or Abel brought an offering. Time doesn't not seem to be the differentiator here. I believe that the Bible clearly gives us two different reasons that God didn't approve of Cain's offering.
Hebrews 11:4 gives us one obvious explanation.
"By an act of faith, Abel brought a better sacrifice to God than Cain. It was what he believed, not what he brought, that made the difference. That's what God noticed and approved as righteous."
Ephesians 2:8 tells us that we are saved by grace, through faith. Romans 3:22 (and many other verses) tells us that our righteousness comes through faith in Jesus. And Galatians 2:16 assures us that we are justified not by what we do (Cain), but by what we believe (Abel). If we, like Cain, attempt to be accepted by God based on what we do rather than what we believe, we will be disappointed.
Secondly the Bible also teaches us in Hebrews 9:22 that without the shedding of blood there is no forgiveness.
"[In fact] under the Law almost everything is purified by means of blood, and without the shedding of blood there is neither release from sin and its guilt nor the remission of the due and merited punishment for sins."
Cain did not follow God's instructions. His offering did not involve blood, so it was not accepted. The most expensive gift in the world could be sacrificed to God, but it was and is only by the blood sacrifice that sins were, and are forgiven. (i.e. Jesus)
The story of Cain and Abel is a story of faith and atonement. It is not a story of tithing.