The doctrine of tithing is a tricky subject - no doubt. It is a deeply personal and private matter between an individual and God.
The Intercontinental Church of God, of which Garner Ted Armstrong was founder, teaches tithing. The fullest exposition of the doctrinal basis for the teaching by the ICG (and the Garner Ted Armstrong Evangelistic Association) of Biblical tithing is found in the ICG Bible Correspondence Course , Lesson 20 (intercontinentalcog.org/ICGCC/Lesson_Twenty.php).
In this sermon Garner Ted Armstrong expounds from the Bible the basis for the doctrine of tithing.