Buemi had been disqualified from Saturday's race
for a tire pressure infringement, but his luck turned 24 hours later when he was gifted a win after Felix Rosenqvist, who started from pole and led from start to finish, was hit with a 10-second penalty.
Calamity struck for the Swede at the midway point during the change of cars as Rosenqvist almost collided with teammate Nick Heidfeld in the pit lane.
The pair somehow managed to avoid one another, but Rosenqvist didn't escape unscathed as race stewards gave him a 10-second time penalty for an unsafe release.
The time was added at the end of the race and despite crossing the line first, he wasn't far enough in front of Buemi who finished meters behind.
The reigning world champion admitted to feeling "lucky" after the race, but will be quietly pleased that he has been able to consolidate his championship lead over di Grassi who ended up finishing third after starting seventh on the grid.
"I'd like to congratulate Felix because I feel like he's the real winner -- I got a little bit lucky," Buemi said afterwards.
"I tried to leave him a bit of space so he would finish 10 seconds ahead of Lucas," the Swiss added, referring to his race strategy after being alerted to the time penalty over team radio.
For Rosenqvist, who had celebrated his first Formula E win on Saturday, joy had turned to despair.
"I felt like it was an awkward situation. I think it was just something that happens," Rosenqvist said.
"I was actually quite surprised we were able to keep in front of him."
The result leaves the championship finely poised. Buemi now holds a 32-point lead over di Grassi but the Swiss will miss the next two races in New York due to prior commitments to the Toyota Gazoo Racing team in the World Endurance Championships.
Despite a bittersweet Berlin weekend, Rosenqvist moves up to third in the standings ahead of Buemi's Renault eDams teammate Nico Prost.