Youngster Simon Spender was shown a red card by Lincolnshire based referee Darren Driesdale for a two-footed tackle on United's Karl Hawley. Other players joined in a skirmish around the Wrexham man before the referee pulled him clear.
It was a blow to Wrexham, made worse by the resulting free kick which cost the goal.
Adam Murray, hoisted the ball into the goalmouth where goalkeeper Michael Ingham missed it as it ran out for Brendan McGill to prod it in at the back post.
Wrexham could have levelled in the last minute when Kevin Gray was penalised for holding Dennis Lawrence but goalkeeper Anthony Williams threw himself across and turned Jonathan Walters spot kick around the post.
Wrexham had an earlier penalty appeal turned down when Lee McEvilly was blocked by Gray and Danny Livesey as he broke into the area.
He prodded the ball against the foot of the post as he went down and the referee waved penalty appeals away.
Earlier Walters was blocked just in front of goal when clean through and then Walters saw Livesey clear his shot off the line.
Lee McEvilly hit another shot straight at the goalkeeper before a Darren Ferguson corner picked out Lawrence for a firm header that the keeper knocked into the air before tipping it over the bar.
Manager Paul Simpson who was organising the Blues from midfield tried a shot that flew to high as did another from Hawley. Another chance went begging when Raphael Nade tapped a neat pass across the box where Simpson might have added another goal for his side but Shaun Pejic cleared.
Both sides had started defensively with Wrexham playing five men across the back before the game burst into life with United's Derek Holmes bursting into the area' only to be foiled by Inghams flying saved.