hafa at varðveita "to have in [one's] keeping" (cf. post #6449).

> En nú haf þú njósn af nær er þeir koma til bæjarins og seg mér."
> But now you have watch of when they come to the town and tell me.'

nær er "when" (Zoega's nær (7) + the relative 'er', which can also
mean "when" on its own).

MM & HP: "Keep watch and let me know when they arrive in town."
More literally perhaps: "But now get news/intelligence of [them] when
they come to town and tell me." According to the dictioanries 'njósn'
can be either the action "spying, scouting, keeping watch" or the
result "news, intelligence". Maybe both are implied here?

LN