Neither Cain or Abel were following God’s command. If there was an actual sacrifice, it was something that Abel did on his own, but there is also evidence that there may not have been a sacrifice at this time, but it did start sometime which is evidenced by the fact that even after the flood Noah sacrificed some of the animals on the ark, which led God to remark that mankind is evil from his youth (Genesis 8:21).
This is also some of the pre-Hebrew sacrificial system that was disbursed throughout the Middle East region that Abraham followed throughout his life that we mentioned previously..
In the Love of the Lord,
Frank and Mary