"The Reubenites and the Gadites owned cattle in very great numbers"
In this week's parashah, the Gadites and the Reubenites ask to stay on the "wrong side" of the Jordan to accommodate their many cattle (Numbers 32:5). According to Numbers Rabbah 22.8, these cattle were seized from the Midianites (Numbers 31:9). The Torah itself does not report this, but describes in detail how the Midianite booty was meticulously turned over to Moses for proper distribution. So what justifies the claim in Numbers Rabbah?
An interpretative principle at play here seems to be juxtaposition. Since the Gadite and Reubenite cattle are mentioned immediately after the disposal of Midianite booty, surely they are one and the same?
A more interesting justification is typology - repeating patterns. In this case, the pattern that repeats is that external wealth causes internal conflict. Abram leaves Egypt with many cattle. As a result, the land cannot support Abram and his nephew Lot, and they separate. The gold and silver that the Israelites take out of Egypt is used to make the Golden Calf, causing conflict between Moses and Aaron.
This pattern must be distinguished from Israel's organic growth, which generally leads to eviction by a foreign power (Exodus 1:7, Numbers 22:3, Deuteronomy 26:5). Though painful for those affected, this is a step towards a land of their own. Wealth acquired from "imported goods" is a step away. As Numbers Rabbah also claims, the Reubenites and Gadites were the first tribes to be exiled (I Chronicles 5:26), because they snatched wealth and separated themselves from their brothers on account of it.
Even without this typological reading, we see that the request of these two tribes to separate themselves was misguided. Read in the light of accounts of goods brought out of Egypt, we understand that the problem began with the wrong sort of wealth.