The principle of the tithe and giving is not OT/NT, it predates the law, it goes all the way back to the garden.
Whenever, Jesus re-articulated a principle in the Gospels, He always raised the standard, He never lowered it. He also made it about the heart vs solely obligation. The point is, can you give according to God's standards and still do it cheerfully? Versus, give an amount that keeps you cheerful (ie, doesn't bother you)? This is because God knows that the #1 thing that can steal our hearts from Him is the love of money. If we can be obedient AND give cheerfully, that demonstrates He has our heart, not money.
I have heard the arguments time and time again that tithing is not NT. And this is what I have seen first hand when it comes to giving. Those that argue that tithing is not NT, never give even close to 10%, not even here or there. Those that do tithe are often generous beyond 10%, whether that be to the church, homeless, personal blessing to their pastor, or whatever. So, which one sounds more in line with the heart of God?
btw, I am not talking about mega-church, televangelists sort of thing, that is a different discussion entirely. I'm talking about your average everyday local church which - despite the frequent accusations to the contrary - are not fleecing anybody, but rather trying to keep the lights on, help as many as they can, and provide for their family who already have to sacrifice more than most people realize.