The Sulphurites announced at 11.15pm on Friday they had sold their top-scorer of the last two seasons. The Dragons were more circumspect.
Although not disclosed, the fee was thought to be a record for the fourth tier, somewhere in the region of £500,000.
Harrogate reinvested immediately, signing Stevenage's Josh March as a replacement.
But the Football League announced the next morning Armstrong's registration had not been completed in time. March's was.
Football League clubs are not allowed "deal sheets" to gain a two-hour extensions, as Bournemouth and Leeds United did when they swapped wingers Jaidon Anthony and Luis Sinisterra in loans announced at midnight. Other deals involving top-flight clubs went later still.
Wrexham had been seeking a replacement for Paul Mullin after he punctured a lung playing against Manchester United during pre-season.
Harrogate manager Simon Weaver had always maintained he would not sell Armstrong without a replacement, despite the player making himself unavailable for the first two matches of the season.
"A lot of work has gone in to identify Luke and to get him to the club," said Wrexham and former Bradford City manager Phil Parkinson.
"But for whatever reason that didn't get over the line and we're disappointed.
"We put offers in and worked really hard to get a deal that was really suitable for Harrogate and us.
"It was held up because Harrogate wanted to get players in themselves."
Wrexham are now weighing up whether to appeal but if not Armstrong will return to Wetherby Road until at least January.
Meanwhile, the club begin their Football League Trophy campaign at home to Nottingham Forest's Under-21s on Tuesday.
The group stage is notorious for fan apathy, and especially towards the junior teams taking part, but striker James Daly says it is important to take it seriously.
"All these competition are great to work on our shape and for opportunities to gain confidence we can take into the league,” he argued.
"It's massively important we get off to a good start and try and progress as far as we can.
"It's a massive opportunity to go and win and get a bit of confidence going into the Gillingham game."