The tithe under the Law was different from the tithe that Abram had given before the Law. Abram tithed spoils of war, the Law required agricultural produce and livestock.
The tithe under the Law was not given to the Priests of Israel, it was given to the Levites. Nehemiah 10:37-38 reveal that the Priests had to be with the Levites when the Levites received the tithe from the children of Israel.
The Levite, in turn was to take a tithe of the tithe to the House of God. The children of Israel did not tithe to the house of God, they took their tithe to the Levites in their cities.
It was not the children of Israel who had robbed God in Malachi 3. No, it was the Priests of the Temple who had robbed Him.
I encourage you to read the entire book of Malachi. It is only four chapters. But, notice Malachi 2:1.
"And now, O ye Priests, this commandment is for you."
What commandment is God speaking of through Malachi the Prophet? There is no commandment at all in Malachi 2.
It is not until you get to Malachi 3:7 that you find a commandment....
"Return unto me". Return? How? "Will a Man Rob God?" How have we robbed you? "In tithes and offerings. Bring ye all the tithes into the storehouse..."
God was speaking to the Priests. They had robbed Him. The children of Israel had not robbed Him. They had done their part. They took the tithes as instructed in Nehemiah 10 to the Levites.
The Levites had done their part. They had taken the tithe of the tithe to the Temple. But Nehemiah reveals that the Priests had robbed God. They removed the tithes from the Temple chambers. Read Nehemiah 13:1-12.
Eliashib, the High Priest, removed the tithes from the chambers and moved God's enemy Tobiah the Ammonite in. What he did with those tithes we do not know.
However, God wanted them back. Nehiamiah instructed the tithes be brought back because the Levites were not getting the portion that was due them at the Temple during their work there. They were not being sustained through the tithes that they had brought to the Temple with them because the tithes had been stolen by the Priests.
Judah, a Levite, went out into Jerusalem and Israel to collect tithes. Now, the people had already given their tithes to the Levites previously. But here was the Levite collecting tithes a second time. The people were being robbed! They had given their tithes and now were expected to give more.
Judah brought the tithes he collected to the Temple chambers. But this was not the tithe that had been stolen from the chambers. This was tithes that were taken from the people to replace the stolen tithes. God's stolen tithes were still missing.
And God wanted them back. Hence, the commandment and subsequent instruction in Malachi 3. The Priests who had stolen the tithe were to bring the tithe back. The Priests were guilty of robbing God by removing the tithes, and the people were robbed by having to pay a second tithe that year.