Why use acre instead of acres in the sentence, "The 10 acre property was transferred to the city"?

I'm pretty sure it's because when you say The [10 acre] property, 10 acre here is acting as a kind of adjective describing property. When you say The property is ten acres, acres here is the plural form of a noun and ten is the adjective describing it.

When you are using acre as a unit of measurement it's not plural but when you are talking about acre as "lands" or "property" then it is plural.

Dictionary.com:

a·cre
–noun 1. a common variable unit of land measure, now equal in the U.S. and Great Britain to 43,560 square feet or 1/640 square mile (4047 square meters).
2. acres, a. lands; land: wooded acres.
b. Informal. large quantities: acres of Oriental rugs.

http://dictionary.reference.com/browse/acre

This seems to be the case with all units of measurement, not just acre; feet, meters, grams, etc.

Six inch ruler, 10 pound note - the (six inch) is an adjective (OK, an adjectival phrase) and is one unit. The ruler was six inches long - it really was, I counted all six of them; they are six units.

Apart from these logical explanations, the English language can seem illogical and irrational at times, because the rules are drawn from widely differing sources,