"which was to be delivered in unmarked five dollar bills" is the subordinate sentence, since "the note repeated the telephone..." is the main one.
Like any clause that begins with a relative pronoun ("which" in this case), it cannot stand as a sentence on its own; it depends on the noun "ten thousand dollars" because it acts as a modifier of it. This function of modifying a noun applies only to adjectives, so this is an adjective clause.
The subordinate clause in the sentence is "which was to be delivered in unmarked five dollar bills," and it functions as an adjective clause. It provides additional information about the noun it follows. This classification is due to its role in modifying a noun, which is typical of adjective clauses.
;