Talking about the law, there are many things which we are not practicing anymore because we are not under the dispensation of the law of Moses. Moses gave the children of Israel the law to follow when they were coming out of the land of Egypt. The land of Egypt was a land of slavery. God gave the laws to Moses so that he could be able to establish a way in which the people could come to God. The law was not everything perfect on it own. The perfection of the law is found in the Lord Jesus Christ. So, when the Lord Jesus Christ came, we have been redeemed from the tutorage of the law. The law does not decide what we do anymore.
Tithing is something that happened even before the law of Moses. It was in the days of Abraham that tithing was first recorded. That was many many years before the law. Abraham first give tenth part of what he had to Melchizedek, the Priest of Salem. Thereafter, we saw Jacob also gave tithe when he promised God that if he will take him to where he was going and brought him back, he will give the tenth part of his possession to God.
In the New Testament, we are not commanded to tithe as the Old Testament people were commanded. We give whatever we please to God willfully.