How the financial sense of "capitale" derived from the adjective "capitale" as something which has to do with head?

Question

Unlike Ayto below, Etymonline doesn't mention any Italian etymons for the English "capital" meaning wealth.

capital [13]

Etymologically, capital is something that is at the top or ‘head’; it comes from Latin caput ‘head’. The various current English uses of the word reached us, however, by differing routes. The first to come was the adjective, which originally meant simply ‘of the head’ (Milton in Paradise lost wrote of the Serpent’s ‘capital bruise’, meaning the bruise to its head); this came via Old French capital from Latin capitālis, a derivative of caput. The other senses of the adjective have derived from this: ‘capital punishment’, for instance, comes from the notion of a crime which, figuratively speaking, affects the head, or life. Its use as a noun dates from the 17th century: the immediate source of the financial sense is Italian capitale. The architectural capital ‘top of a column’ (as in ‘Corinthian capitals’) also comes from Latin caput, but in this case the intermediate form was the diminutive capitellum ‘little head’, which reached English in the 14th century via Old French capitel.

Word Origins (2005 2e) by John Ayto. p 92 Right column.

Why "capitale" started to have the finance related meaning it has in Italian?

Answer 1

As you have found in the dictionary, the adjective capitale means something which has to do with head, considered the main part of a body, as explained by Treccani dictionary

in quanto il capo, cioè la testa, è la parte principale e più nobile del corpo

This is the definition of the adjective capitale from Grande dizionario della lingua italiana:

Che riguarda il capo, la vita di una persona (pena, sentenza, supplizio, peri­colo capitale)

From this, the following figurative meaning arised:

Principale, essenziale, di fondamen­tale importanza.

I.e., it's an adjective used to qualify something essential or of great importance. For instance, the most important city of a region or state is called "la città capitale" or simply "la capitale".

The first attested use of the noun capitale with a financial sense is this sentence coming from the Testi fiorentini, a collection of tests from 1211 to 1313:

S’elli non pagasse, sì no promise di pagare Orlandino Galigaio prode e capitale quant’elli istessero.

According to Grande dizionario della lingua italiana, the meaning of capitale in this sentence is

La parte principale di un patrimonio in denaro: rispetto alla somma mi­nore costituita dagli interessi che essa produce.

That is,

The main part of a money asset compared to the lesser amount constituted by the interest it produces.

The idea is that this "capitale" is the main part of an amount of money in the same way that the head is considered to be the main part of a body.