Six months after a shock diagnosis that saw him miss the entire first half of the season, Haller finally opened his account for the club he joined last summer.
"I've been waiting for this moment since day one. When I scored the goal, it felt like the whole stadium was on fire," Haller told Sky.
"I am happy, but the way is still long, so we will keep walking and hope for more goals," he added. Freiburg was reduced to 10 men early in the first half after French full-back Kiliann Sildillia was booked twice in the space of two minutes.
Former Freiburg player Nico Schlotterbeck then heaped more misery onto his old club, his low snapshot beating goalkeeper Mark Flekken at the near post to give Dortmund the lead.
Schlotterbeck and Niklas Suele handed Freiburg a lifeline with some farcical defending on the stroke of half-time, allowing Lucas Hoeler to sweep in an unexpected equalizer.
Meanwhile, in the capital, Union Berlin was working on their own fairytale as a late winner from US international Jordan Siebatcheu sent the high-flying minnows top of the table.
'Ironic'
Former Union player Marcus Ingvartsen had canceled out Kevin Behrens' first-half opener with a late penalty, but the Berliners rallied to claim a fourth straight victory in 2023.
The win had Union fans singing about winning the title, but captain Christopher Trimmel said his side was keeping their feet on the ground.
"I was singing along, but only ironically," he told Sky. "There's still a long way to go this season and we are not going to start talking about the title." Eintracht Frankfurt also remained within four points of the top as France forward Randal Kolo Muani scored twice in a 3-0 win over hapless Hertha Berlin.
Having put Frankfurt ahead from the penalty spot, Kolo Muani ghosted through the Hertha back line to make it 2-0 before the half-hour mark. Aurelio Buta added a third on the stroke of full-time.
RB Leipzig's 0-0 draw at Cologne meant that Frankfurt remain a point adrift of the Champions League places in fifth.
Hertha has slumped to four straight defeats since the beginning of 2023, and are now five points adrift of safety after fellow strugglers Bochum cruised to a 5-2 win over Hoffenheim.
Philipp Hofmann gave Bochum a deserved lead with a tap-in before Philipp Foerster and Takuma Asano showed up a lifeless Hoffenheim defense to make it 3-0 by half-time.
Erhan Masovic and Moritz Broschinski added two more for Bochum after the break, with Christoph Baumgartner and Munas Dabbur grabbing consolation goals for the visitors.
Bochum's win plunged neighbors Schalke further into the mire, as they battled to a second successive goalless draw away to Borussia Moenchengladbach in Saturday's late game.
Despite a spirited attacking display, Thomas Reis' side failed to take their chances and remain rooted to the bottom of the table.