Archaism in legal language

Demosthenes (23.28, 33) quotes and explains a law about homicide.

Τοὺς δ’ ἀνδροφόνους ἐξεῖναι ἀποκτείνειν ἐν τῇ ἡμεδαπῇ καὶ ἀπάγειν, ὡς ἐν τῷ <αʹ> ἄξονι ἀγορεύει, λυμαίνεσθαι δὲ μή, μηδὲ ἀποινᾶν, ἢ διπλοῦν ὀφείλειν ὅσον ἂν καταβλάψῃ. εἰσφέρεινδ’ ἐ<ς> τοὺς ἄρχοντας, ὧν ἕκαστοι δικασταί εἰσι, τῷ βουλομένῳ. τὴν δ’ ἡλιαίαν διαγιγνώσκειν.
...
‘λυμαίνεσθαι δὲ’ φησὶ ‘μή, μηδὲ ἀποινᾶν.’ ταῦτα δ’ ἐστὶν τί; τὸ μὲν δὴ μὴ λυμαίνεσθαι γνώριμον οἶδ’ ὅτι πᾶσιν μὴ μαστιγοῦν, μὴ δεῖν, μὴ τὰ τοιαῦτα ποιεῖν λέγει, τὸ δὲ μηδ’ ἀποινᾶν μὴ χρήματα πράττεσθαι· τὰ γὰρ χρήματ’ ἄποιν’ ὠνόμαζον οἱ παλαιοί. 

"... for past generations used to call such money [in Loebssprache] 'amercement'."

ἄποινα is rare outside the Iliad. An instance that is not in a text relating the Trojan Cycle is Plato Laws 862c, again a legal context, which is also the only other occurrence in Attic prose.