Weidenfeller called time on his Dortmund career at the end of the 2017/18 season, bringing to an end a 16-year association with the club that he joined from Kaiserslautern in 2002.
Seven of those years were spent in the company of Klopp, who managed Dortmund between 2008 and 2015, leading them to two Bundesliga titles and a Champions League final.
Weidenfeller was Dortmund's first choice goalkeeper throughout that time, making more than half of his 453 appearances for BVB under Klopp's stewardship.
The two men will be reunited at the Westfalenstadion one last time on 7 September, as Dortmund revealed in a video which was recorded before Liverpool played Dortmund in a pre-season friendly in Charlotte last week.
In the video, Klopp approaches Weidenfeller and says wistfully: "I'd love to play again", to which the 37-year-old responds: "I've got a great idea. Why don't you come along to my farewell match on 7 September?"
"Anyone who wants to see Klopp and Weidenfeller in action together will have to get a move on because we're not the youngest anymore," beams the Liverpool boss. "It's going to be a 'grandios' testimonial."
The testimonial will take place during the first international break of the season, with Michael Ballack and Torsten Frings among the biggest names confirmed so far.