The context is:
Si quis hominem ingenuum qui lege Salica vivit, in hoste in companio de companiei suorum occiderit, secundum quod in patria, si ipse occisus esset, componere debuisset, in triplo componat.
Since it clearly is connected to de, it stands to reason that the word must be an ablative, and the only nouns having an ablative ending in -i are third declension i-stem nouns.
So what word is it, what does it mean, and what does the whole sentence mean? The answer is apparently companies, which is at least extremely rare, and possibly exists nowhere else; however, du Cange lists it:
Contubernium, Gall. Compagnie. Vide Companium
And lo, for companium this precise sentence is the first example. Du Cange translates this as: in contubernio ac societate suorum contubernalium and remarks: Galli dicerent, En la compagnie de ses Compagnons. The dictionary does not elaborate on what we should expect other forms of this word to look like (e.g. nom. pl.), which frankly is a mystery to me.
(Another mystery which du Cange does not shine any light on either is the strange apodosis of the rule: I have to pay a fine if I am killed in my own country? How does that happen?)
It is remarkable that several rather different versions of this law have been found. On Google Books, a German annotated edition of the Lex Salica gives, among others, this variant:
Si quis in hoste de companio de conpagenses suos hominem occiderit, secundum quod in patria si ipso occidisset componere debuisset in triplo componat
... and, with respect to this version, remarks:
Die oben angeführte Nov. 177 verdunkelt den Sinn durch ihr erbärmliches Latein. Sie will sagen: Wenn Jemand auf einem Feldzuge Einen von seinen Landsleuten aus seiner Compagnie (Gefährtschaft) tödtet, so büsst er eine dreimal so grosse Strafe, als er hätte büssen müssen, wenn er denselben in der Heimath getödtet hätte.
The above-mentioned Nov. 177 obscures the meaning due to its miserable Latin. It is trying to say: If somebody on a military campaign kills one of his compatriots from his company (fellowship), he must pay a fine thrice as high as if he had killed the same person at home.
Interestingly, in this reading it refers to a killing where the victim is part of the perpetrator's military unit. I mention this not least because if Frankish scribes (or whoever) wrote things like "de compagenses suos," then on what basis do I claim that "it must be an ablative" ...?