"I always thought that "tithing" was part of Mosaic Law... that it had no place in the Grace administration... that Jesus only referred to it a couple of times because he was living under Mosaic Law at the time... "
About your statement above -
What was taught when I was in twi was that tithing was instituted with Abraham which was before the Mosaic law, so it transcended the law and still was God's will after the law administration.