The church is guided by the doctrines of Jesus Christ. When Jesus was teaching the people, he answered their questions and enlightened them in many issues of religious activities. The church was born as a fulfillment of the law. The church has its doctrines which is what is guiding the activities and religious activities.
In the Old Testament, the children of Israel were guided by the laws of Moses. The law governed most of the things which they were doing. Their behavior and interaction with others were guided by the law. The law prescribed how they should give their offerings. There were so many kinds of offerings in the Old Testament. Each of them had methods for offering them.
Tithing wasn't something that started in the law. It was there since the days of Abraham. Our father Abraham paid tithe to Melchizedek. Jacob also promised to pay tithes if God would take him to Laban house and bring him out. So, tithing isn't altogether of the law, it only reflected in the law.