וַיֹּאמְר֞וּ לַמַּלְאָכִ֣ים הַבָּאִ֗ים כֹּ֤ה תֹֽאמְרוּן֙ לְאִישׁ֙ יָבֵ֣ישׁ גִּלְעָ֔ד מָחָ֛ר תִּֽהְיֶֽה־לָכֶ֥ם תְּשׁוּעָ֖ה כְּחֹ֣ם הַשָּׁ֑מֶשׁ וַיָּבֹ֣אוּ הַמַּלְאָכִ֗ים וַיַּגִּ֛ידוּ לְאַנְשֵׁ֥י יָבֵ֖ישׁ וַיִּשְׂמָֽחוּ׃
The messengers who had come were told, “Thus shall you speak to Jabesh-gilead’s side*: Tomorrow, when the sun grows hot, you shall be saved.” When the messengers came and told this to the representatives of Jabesh, they rejoiced.
*In the parley with the Ammonites.
(The above rendering, with footnote, comes from the RJPS translation, an adaptation of the NJPS translation. Before accounting for this rendering, I will analyze the plain sense of the Hebrew term containing אִישׁ.)
A message is being sent. As a label for its recipient, the singular form of אִישׁ implies that the message’s content has strategic import in the negotiation between the two opposing sides. A further implication is that the pending rescue mission must be kept hidden from the other side. (The plural form later in the same verse refers to the same recipients, but it regards them in terms of their affiliation with the town, rather than their situatedness in the parley.) In short, a grammatical feature (number) is deployed here in an unusually meaningful manner.
On the “collective” usage of the singular אִישׁ in the context of a parley between two parties, see my comment at Josh 9:6.
As for rendering into English, the NJPS ‘the men of Jabesh-gilead’ recognizes a collective usage, but without a situational orientation. See my comment at Josh 10:24.