Thomas was left in the clear to curl past Jacob with his left foot for 3-3. Luke Hendrie had one shot saved by Dixon, van Velzen slipped with the goal at his mercy in the closing stages and Kenji Gorre also worked the Black Cats' keeper as United were unable to regain the lead.
With Barmby receiving treatment after being replaced, there could be a doubt over the top scorer's involvement in Friday's game at Old Trafford and captain Luke McCullough was also substituted towards the end.
United: Jacob; Grimshaw, Dalley, McCullough (Rowley 88), Blackett; Barmby (Gorre 65), Rudge, Pearson, Daehli (Hendrie 65), van Velzen; Wilson. Subs not used: Coll, Wilkinson.