Let me take a crack at it. Chapter 12 of Deuteronomy,
verse one says . . .
Bible, King James Version
Deut.12 Verses 1
[1] These are the statutes and judgments, which ye shall observe to do in the land, which the LORD God of thy fathers giveth thee to possess it, all the days that ye live upon the earth.
What are these?
They are statutes (laws).
Where are they to observe or do these laws?
In the land.
These statutes were to be observed in the land that God had given them. After verse one, Moses begins to list the statutes that they are to follow in the land. In verse six we encounter the tithe.
Bible, King James Version
Deut.12 Verses 6
[6] And thither ye shall bring your burnt offerings, and your sacrifices, and your tithes, and heave offerings of your hand, and your vows, and your freewill offerings, and the firstlings of your herds and of your flocks:
Does this mean that the Israelites were to ONLY observe the statutes that follow verse one in the land?
Well . . . it appears that way to me. If God had wanted them to follow the statutes wherever they happened to be, why would He say to observe to do them "in the land"?
What do you think Esaias?
I think this is a weak and insufficient argument in proof of tithing being limited to the land of Israel. Whether tithing is or is not limited to the land of Israel, the argument you presented does not prove it.
The reason is your proof proves too much. The statutes and judgments which were to be done in the land include for example the following:
Not eating blood (ch 12 v 16, 23-25)
Avoiding syncretism and paganism (v 30-31)
Not altering the Law (v 32)
Not following false prophets (ch 13 v 1-4)
Not eating unclean meats (ch 14)
Alms (ch 15 v 7-8)
Not bribing judges (ch 16 v 19)
Not setting up idols (ch 16 v 22)
Not practicing occultism (ch 18 v 10-12)
Not cross dressing (ch 22 v 5)
And many more. Yet, if your reasoning were correct, Israelites could violate all these rules (and more) as long as they were outside the land of Israel.
Therefore, the tithe being limited to that which was produced within the land of Israel would have to rest on some other basis than God saying "these are the things you will do in the land I give you".
The phrase "which you shall observe to do in the land" is not restrictive, anymore than a parent telling a child "in THIS house you will show good manners" is an authorization for a child to be ill-mannered at the neighbor's house. Which it would be if it were merely restrictive. Hope that makes sense.