You are correct. ἐχουσία is not a valid word.
Thank you for also catching my misidentification of οὐσία as the present participle of εἰμί.
My reply somehow was posted while I was working on Aristotle's problematic use of οὐσία and his distinct use of δύναμις, not ἐξουσία, to indicate potentiality (Cohen, S. Marc, "Aristotle's Metaphysics", The Stanford Encyclopedia of Philosophy (Summer 2014 Edition), Edward N. Zalta (ed.), URL = <Aristotle's Metaphysics (Stanford Encyclopedia of Philosophy/Summer 2014 Edition)
In Metaphysics Ζ, Aristotle introduces the distinction between matter and form synchronically, applying it to an individual substance at a particular time. The matter of a substance is the stuff it is composed of; the form is the way that stuff is put together so that the whole it constitutes can perform its characteristic functions. But soon he begins to apply the distinction diachronically, across time. This connects the matter/form distinction to another key Aristotelian distinction, that between potentiality (dunamis) and actuality (entelecheia or energeia). This distinction is the main topic of Book Θ.
Aristotle distinguishes between two different senses of the term dunamis. In the strictest sense, a dunamis is the power that a thing has to produce a change. A thing has a dunamis in this sense when it has within it a “source of change in something else (or in itself qua other)” (Θ.1, 1046a12; cf. Δ.12). The exercise of such a power is a kinêsis—a movement or process. So, for example, the housebuilder's craft is a power whose exercise is the process of housebuilding. But there is a second sense of dunamis—and it is the one in which Aristotle is mainly interested—that might be better translated as ‘potentiality’. For, as Aristotle tells us, in this sense dunamis is related not to movement (kinêsis) but to actuality (energeia)(Θ.6, 1048a25). A dunamis in this sense is not a thing's power to produce a change but rather its capacity to be in a different and more completed state. Aristotle thinks that potentiality so understood is indefinable (1048a37), claiming that the general idea can be grasped from a consideration of cases. Actuality is to potentiality, Aristotle tells us, as “someone waking is to someone sleeping, as someone seeing is to a sighted person with his eyes closed, as that which has been shaped out of some matter is to the matter from which it has been shaped” (1048b1–3).
Since Aristotle was talking extensively about the nature of οὐσία and what derives
from it, it seems odd that he would use δύναμις, not
ἐξουσία, to indicate "potentiality," if ἐξουσία meant "a permitted being" or "'a being possible/permitted,' i.e. 'a (state/condition/process of) being possible/permitted' — in other words, a potential or potency."
In addition, apparently, not everyone believes that οὐσία "by his [Aristotle's] time, had already come to mean 'property,'" at least not exclusively. Citing the same source:
...the Categories begins with a strikingly general and exhaustive account of the things there are (ta onta)—beings. According to this account, beings can be divided into ten distinct categories. (Although Aristotle never says so, it is tempting to suppose that these categories are mutually exclusive and jointly exhaustive of the things there are.) They include substance, quality, quantity, and relation, among others. Of these categories of beings, it is the first, substance (ousia), to which Aristotle gives a privileged position.
Substances are unique in being independent things; the items in the other categories all depend somehow on substances.
In addition to "that which is one's own, one's substance, property
," "real property, immovables
," "freq. of estates
in Egypt," Liddel and Scott give additional definitions for οὐσία: "in Philos., like Ion. φύσις (with which it is interchanged in various uses" has the submeanings of "stable being, immutable reality
," "substance, essence
," "true nature
of that which is a member of a kind," "the possession of such a nature, substantiality
," "in the concrete, the primary real
, the substratum underlying all change and process in nature," "in Logic, substance
as the leading category," (Liddell, Henry George; Scott, Robert. A Greek-English Lexicon. οὐσί-α http://www.perseus.tufts.edu/hopper/text?doc=Perseus:text:1999.04.0057:entry=ou%29si/a
They also give the following definitions for ἐξουσία: "power, authority to do a thing," "abuse of authority, licence, arrogance," "poetic licence [sic]," "office, magistracy," "abundance of means, resources," and "pomp." (http://www.perseus.tufts.edu/hopper/morph?l=e%29cousi%2Fa&la=greek&can=e%29cousi%2Fa1&prior=magistrates#lexicon
It seems to me that the meaning of ἐξουσία reflects something that which one has or is. Even if ἐξουσία is derived from ἐκ + εἰμί, rather than from ἐκ + οὐσία, its uses seem to extend beyond "authority," "power," or even "potential," as we understand them, and include "abundance of means, resources," and "pomp," both of which are at least logically related to the noun οὐσία. Could the meaning of ἐξουσία at least have been influenced
by οὐσία? Is it unreasonable to think that it may have at least included
the meanings "the property of," or "that which belonged or pertained to," even if the latter can include the "essence" of someone or something?