These two have no overlap in meaning whatsoever, which is unusual — most confusable pairs are at least cousins. "Principal" and "principle" are unrelated in sense and merely happen to sound the same. That makes the error especially visible when it lands in something formal, and it lands in formal writing more than most, because both words belong to contracts, finance and academia.
As a noun: the head of a school, the main party in a transaction, or the original sum of a loan. As an adjective: main or most important.
A noun only. A fundamental truth, rule, or standard of behaviour.
Quick way to remember
Remember that a principal is your pal — a person. And a principle is a rule, both ending in "-le." If the word is describing a person, an amount of money, or means "main," it takes the "a."