Exegesis:

ei de mēge scil. dunatos estin ‘and if (he is) not (able),’ ‘if he cannot.’

eti autou porrō ontos ‘while he is still far away.’ autou refers to the other king. porrō also 24.28.

presbeian aposteilas erōta ta pros eirēnēn ‘he sends an embassy and asks for terms of peace.’ For apostellō cf. on 1.19. ta pros eirēnēn lit. ‘that which belongs to/has to do with peace,’ hence ‘terms of peace.’

presbeia (also 19.14) ‘embassy,’ here abstract for concrete, ‘ambassadors,’ ‘envoys.’

Translation:

While the other is yet a great way off, or, “long before the enemy approaches” (New English Bible), may better be transposed after the first or the second verbal phrase of the main clause, with ensuing adjustments.

Sends an embassy (or, envoys/messengers) and asks, or, ‘orders some representatives (or, some spokesmen, cf. Kele; or, ask some of his servants/officers/counsellors) to go (to the other king) to ask.’

To ask terms of peace, or, ‘to beg agreement about peace’ (Yao), ‘to ask about the becoming-good-with-each-other again’ (Tae’); or, more descriptively, ‘to ask on what conditions he (i.e. the other king) will make peace’ (cf. An American Translation), ‘to ask, “What must we (exclus.) do in order that there may be peace, or, that the war may come to an end?” .’ Some idiomatic phrases used are, ‘to ask to be friends with each other’ (Thai 1967), ‘to ask-ask-as-a-favour time good’ (Pohnpeian), ‘to make his heart willing-to-be-reconciled in order that thus will be calmed down the trouble started by them’ (Tzeltal, combining two such phrases). For peace cf. also 1.79.

Quoted with permission from Reiling, J. and Swellengrebel, J.L. A Handbook on the Gospel of Luke. (UBS Handbook Series). New York: UBS, 1971. For this and other handbooks for translators see here . Make sure to also consult the Handbook on the Gospel of Mark for parallel or similar verses.