You are assuming the Old Testament is no longer valid.
Isa 8:20, To the law and to the testimony: if they speak not according to this word, it is because there is no light in them.
Now I am not even saying that tithing is a law that must be kept.
But I will say that Jesus Christ is the same yesterday, today, and for ever (Hebrews 13:8, Malachi 3:6) and that this means that the OT prophets have a say in our doctrine (according to 2 Timothy 3:16 they do) because it is the same God who wrote the Old Testament who also wrote the New.
This means that when God says the whole nation is cursed with a curse because they are withholding tithes, that this applies to us today: and when He says that He will pour out a blessing that we will not have room enough to receive it if we will but bring the whole tithe into the storehouse, that this applies to us today.
It is not presented in Malachi as a law; but it is given as a promise, that God will bless with a blessing those who will give to Him His proper due when it comes to their trusting Him in their financial situation.
I don't know if I'm saying things clearly enough tonight as I am somewhat tired tonight and may be beginning to experience burnout from ministry, I don't know. I covet the prayers of the saints on this. Hopefully I will feel better tomorrow.