It's the word "moins/less/menos/meno" that is central here, not the preposition.
So, let's see it first:
Latin: minus/minor (except for "less"), that has a meaning of cutting something, to remove something.
Unless in Latin is "nisi", so it's not the source of the expression I think.
"à moins que", is literally "if you remove that". "à" meaning something like "in this case".
(I don't say it means that literally), in the situation you remove that.
I have no idea if one of those languages modeled the expression of the other one, as the use of the "a" in Spanish is maybe unusual (at least, as far as my level of knowledge in Spanish tells me), either they come from a proto-expression giving this same structure, or one of those languages modeled the expression in the other ones.
The "a" is not a private "a" here, it's absolutely not the same thing that the "un-" like in unless (at least, the modern way to understand it, not the etymology)
So, let's check the unless etymology now:
Unless has a different logics that in those Romance language.
Unless -> old form Unlesse -> older form Onless, is a short for "on a less compelling condition than"
According to this link: https://www.etymonline.com/word/unless
So, we basically compare "a/à" and "on" (not "un").
In this case, in this particular context (and not always), yes, they have the same meaning.
On this case, if, on this condition (we would use "in" not "on", but it's the literal meaning) , à ce prix, à cette condition....
It's all about being positioned "on" a case.
English is not my language, I could try to be clearer in my explanations with a little help.