Goals in either half from Perry Kitchen had United cruising by the 67-minute mark as Bill Olsen's side looked good for a third successive victory.
However, Damien Perrinelle reduced the arrears moments after DC's second to introduce the element of doubt into the hosts' bid, and Sam tapped home with minutes remaining to deny the capital club.
Robles could do nothing about the goal, though, as Kitchen nodded home a Taylor Kemp cross from point-blank range to give the hosts a slender half-time lead.
Both proved costly miss as Kitchen wrapped up his double by curling from outside the area, yet the lead lasted less than a minute as Perrinelle then headed home from a corner to provide an instant riposte.
The goal only sought to spur on the hosts further as Olsen's men finished strongly, with Rolfe and Bobby Boswell missing chances to seal the win.
They were punished, however, as Sam tapped home from close range after Sacha Kljestan's effort was saved.