So what I think you're getting at is that it seems in the fragmented political and judicial landscape of post-Roman Europe, it would be very difficult to safely move revenues collected from one place to another.

And you know what? You're absolutely right. It was incredibly difficult.

The Roman Empire, for all its provincial specificities, was a largely coherent economic area which would not be matched in Europe until the european integration process began in the wake of the Second World War. In the period in between these two very distant moments of unity, european political organizations were very fragmented. I'm going to first examine aristocratic landholding in Europe between more or less the sixth and tenth centuries (a world which I am more or less familiar with) and in a sort of addendum compare this to the very different kind world which the Roman Senatorial Class existed in (a world which I'm less familiar with).

While modern scholarship rejects notions of Rome's catastrophic downfall and instead prefers to examine continuity in many institutions after the Empire's end (and many of my own answers on this sub insist on this) it is important to keep in mind that these continuities were not uniform. In different parts of the former Empire, the governing classes preserved different elements of Roman institutions as they adapted to the needs of their own part of the continent. So the story of the post-Roman transformation is, somewhat surprisingly, both a story of preservation and radical change. And one of the most radical of changes was the inability of landowners to diversify the lands they owned.

Far-flung estate ownership declined rapidly as the Empire unraveled in the fifth and sixth centuries, with the last holdouts (predictably) among what was left of the old senatorial aristocracy in Central Italy, who might be expected to hold estates as far as Sicily in the first half of the sixth century. They too, however, would retrench into more regional holdings as war pitting the Eastern Empire against the Italian Kingdom ravaged the peninsula midway through the 500s. Setting aside the Eastern Empire (where political dynasties could be expected to own land in or near Constantinople in addition to their region of origin) far-flung absentee land ownership became entirely anomalous, only ever practiced exclusively by one or two of the most anomalously wealthy dynasties in a given kingdom. In Italy especially, but also in the urbanized portions of Gaul and Germania, the disappearance of a large governmental superstructure pushed political and social leadership towards local political organizations centered in cities, and as a consequence saw it both desirable and necessary to localize their landholdings. This was, in many respects, part of a broader transformation that would permeate to their very identity of what it meant to be aristocratic in Europe. Indeed, by the eight century it is impossible to trace the lineage of nearly all the most prominent political dynasties in Europe, and yet they can be found exerting their rights and privileges in urban councils much like their Roman forefathers did four centuries earlier. Land localization was as technique which allowed them to adapt to the new political landscape.

Land was wealth and wealth was land, this much is true of any pre-industrial society. But Post-Roman Western Europe is interesting in that land also featured as a building block of state organization. For as much as we can lament the incorrect use of the word "Feudalism," and as much as we dislike the idealized hierarchy of feudal ownership, it is undeniable that the Post-Roman period saw a shift away from the politics of taxation and towards the politics of land. Of course, this was never a total shift, nor was land ownership itself unknown or irrelevant in Ancient Rome: medieval monarchs could and did still raise taxes, just as Roman Aristocrats relied on their land to generate the income they spent in politics. But in the medieval period land ownership takes on a dimension of responsibility and privilege unknown in the Roman period, and this encouraged retrenchment and direct management of estates, discouraging absenteeism. This was only strengthened by administrative officials of the state, such as they could be said to exist, who could no longer rely on taxes as the Roman Proconsuls did in their Provinces; instead they would be asked to rely on whatever they could commandeer from local landowners as well as whatever lands was given directly to them, thus creating a space for the generation of local power bases that they could use to threaten the very political system that appointed them in the first place. If we know nothing of the medieval period, is is that it was very unstable.

While I am a medievalist and as such will spend fewer words on the Roman Empire, a comparison might still be worthwhile. Indeed, it wouldn't be accurate to, as I perhaps have done with my choice of wording above, depict the Roman Empire as the complete opposite of whatever system existed in the period that followed its collapse.

While the Roman period saw both many raw materials and finished goods transit from one end of the Mediterranean to another (mostly consisting of grain the government needed to feed its legions as well as the urban grain dole, with a much smaller but still non-negligible quantity of goods also moved around by individual merchants for sale in the marketplace) there were still factors that discouraged Roman Senators from purchasing estates just anywhere. In fact, the favored regions for senatorial investment were Southern Italy, Sicily, and the Province of Africa (referring to the region around Carthage, that is to say much of modern Tunisia). While there were certainly Roman aristocratic dynasties that also held estates in other parts of the Empire, it was Southern Italy, Sicily, and Africa that consistently housed the core of all the greatest dynasties' landowning portfolios.

We have little information on the specific mechanics by which wealthy Romans collected their agricultural wealth, there are some clear characteristics of the regions where they preferred to invest that we can examine: Southern Italy, Sicily, and Africa are all located on a straight geographic axis, making it easy to move people and resources between them; they were also heavily romanized, with well-established institutions upholding Roman laws (and Senatorial privileges); and of course it didn't hurt that they were very fertile regions. Thus the clan-like nature of the Roman senatorial dynasties meant that trusted family members could be counted on to oversee these estates without griping about being too far from the capital, so the Pater Familias could trust that wealth generated on these estates could safely stay on the estate itself. Locating estates in highly romanized regions also meant that wealth kept locally could be used to purchase many of the same material luxuries that would be available in Rome. Lastly, good political connections meant that the Senatorial class could easily enter contracts to deliver grain to the dole or to the legions, allowing them to collect payments while staying in the capital. The enormous consumption of goods and raw materials in Rome itself also meant that much of what was produced in a faraway estate could be converted into cash by transporting it to a Roman marketplace, and thus closely overseeing its liquidation. While no method was foolproof, and many goods could certainly be stolen or lost, extensive estate ownership was expressly thought out to compensate: revenues lost in some place would be compensated up by revenues generated somewhere else.

My main source for all that I have posted is Chris Wickham's "Framing the Early Middle Ages," which is not only an immense work of comparative history on the four hundred years after the fall of the Western Empire in Europe, but also does a great job of examining the conditions preceding the Empire's transformation. Regarding estate holding in the Roman period, Wickham cites "Western Aristocracies and Imperial Court, AD 364-425" by John Matthews, in addition to two works by French historians, one published in Italian by François Jacques titled "L'ordine senatorio attraverso la crisi del III secolo," and another (in French) by Mireille Corbier on Proconsular Africa titled, "Les familles clarissimes d'Afrique Proconsulaire."

Regarding instead Late Roman and Early Medieval landholding, Wickham points to "Transformation and Survival in the Western Senatorial Aristocracy, c. A.D. 400–700" by S. J. B. Barnish, and "The Later Roman Empire, 284-602" by A. H. M. Jones. Another source cited of which I am partial to (and clearly biased towards) is the Italian Francesco Marazzi's "Il conflitto fra Leone III Isaurico e il papato e il "definitivo" inizio del Medioevo a Roma."