While browsing in a medieval glossary, I discovered that the word "farm" used to have meanings of which only traces now remain:
"farm
One party might acquire, on a basis similar to a lease, from another party one or more rights, jurisdictions, revenues or properties -- often a combination thereof -- by taking them to farm (today we still talk about "farming out" sources of revenue). A farm was a pre-determined lump-sum amount assessed for one year (although payments might be made in instalments) on an estimation of the value of the revenues. In theory at least, a farm was a fixed, or "firm", amount from year to year, as opposed to a rent, which might fluctuate according to property values; however, in practice farms were often renegotiated if the value of the sources of revenues changed, while "rents of assize" were considered to have a fixed value that could not be changed by, for example, a new owner (such rents themselves being subject to sale, gift or bequest, quite independently from the ownership of the real estate itself). The firma burgi, or fee-farm of a borough, was the annual sum due the Exchequer in return for the king allowing the farmer to administer the borough -- or rather, administer its sources of revenue (which might include property rents and taxes, local tolls, court amercements). Before the period of self-government, boroughs were farmed by local potentates (e.g. county sheriffs) or entrepreneurs; the aim was to be able to make a profit from the revenues beyond the amount due for the farm, and this could result in extortionate measures. Hence the desire of the townsmen to acquire the farm for themselves, which necessitated taking the reins of local administration. With the grants of self-government to boroughs by (principally) kings Richard I and John, the farm became in effect a perpetual lease. The amount of the fee-farm was fixed at the time of the original negotiation, but might increase if subsequent additions to jurisdictions, revenues etc. were granted, or decrease if the value of borough revenues demonstrably decline: thus, for example, we see many adjustments in valuations at the time of Domesday Book, while towards the end of the Middle Ages many towns were making cases for reduction of their fee-farm due to local hardship or abnormal expenses (e.g. building defensive fortifications). Boroughs themselves sometimes farmed out to townsmen sources of revenue (with associated offices, if applicable) or communal properties, since this assured the borough a guaranteed (lump sum) revenue, regardless of what income the farmer was able to extract. "
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