In 2004, Bayern Munich gave Borussia Dortmund a €2 million (£1.3m/$2.4m) loan to help keep the crisis-hit club afloat and pay the players' salaries. Fifteen years later, Bayern have sent another gift Dortmund's way, selling them their best centre-back, albeit at a much higher price than the 2004 loan.
Mats Hummels is the latest arrival at Westfalenstadion this summer as Dortmund attempt to return to the top table of Europe and once again finish first in the Bundesliga standings. Not since the Jurgen Klopp-era have Dortmund taken the title race to the last day of the season, but Lucien Favre's first campaign at the club exceeded all expectations and – for a while – they looked like they might finally be the side to end Bayern's dominance.
Favre is already looking ahead to the new campaign, strengthening the squad with talent all over the pitch. Hummels is the biggest name and most expensive arrival so far this summer, returning to his former home for a club record €38m (£33.8m/$42.5m) to add much-needed experience to the backline.
At the winter break last season, it looked as if Dortmund were on course for glory but injuries in the second-half of the campaign derailed their title ambitions as Favre was often forced to play a makeshift defence, even drafting in midfielder Julian Weigl at centre-back to fill a hole.
Hummels will become the preferred partner of Manuel Akanji, who is only 23. Backup centre-back and stand-in left-back Abdou Diallo is also just 23, while Dan-Axel Zagadou and Real Madrid loanee Achraf Hakimi are both 20. World Cup winner Hummels turns 31 in December, but a loss of pace in the latter years of his career should not be an issue for Dortmund as his game has always been based on more than that. His reading of the play will help snuff out attacks, while his ability on the ball can help BVB build from the back.
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Nico Schulz's arrival will also strengthen Dortmund's defence, having played at both left-wing-back and left-back for Hoffenheim last season. The 26-year-old is entering his prime and will immediately be first-choice at left-back for Favre, just as he is under Joachim Low for Germany. The experience of Hummels and Schulz will not only add stability, but also help the younger players develop as they learn from veterans who have an abundance of Bundesliga experience.
Schulz was one of three new signings Dortmund announced in the week between the end of the league season and the German Cup final. While fellow top-three finishers Bayern Munich and RB Leipzig were preparing for the DFB-Pokal showpiece, Dortmund were doing business. The versatile full-back opened the floodgates for arrivals, with Julian Brandt and Thorgan Hazard also transferring within the Bundesliga.
Brandt was one of Bayer Leverkusen's best players in 2018-19, having converted from a wideman to a central midfielder under new head coach Peter Bosz. The new position allowed him to utilise his ball-playing skills better, giving him more time in possession and therefore more of a chance to dictate games. Former BVB boss Bosz indicated that even before he took over at Levekusen, he envisioned Brandt as a more central player.
“During my time with Dortmund when we played against Bayer in Leverkusen, he was playing on the wing, but I saw him as a midfielder,” Bosz explained to the Bundesliga's official website. “He’s such a good player and you’ve got to give good players the ball as much as possible."
Favre's dilemma could be finding a place for Brandt in an already crowded midfield. Does he sacrifice the more defensive Thomas Delaney to go all-out-attack or push Marco Reus even further forward and play without a recognised forward.
He should have no such problem fitting Hazard into his starting XI, with Raphael Guerreiro entering the final year of his contract and thus likely to leave this summer rather than in 2020 on a free transfer.
Eden Hazard's younger brother was one of the best dribblers in the Bundesliga last season for Borussia Monchengladbach and will form a frightening wing duo with Jadon Sancho, who was the only player with more completed dribbles in 2018-19.
Defences everywhere in the Bundesliga and the Champions League will be wary of the new Hazard-Sancho partnership which could easily become the best wide pairing since Franck Ribery and Arjen Robben's peak years at Bayern Munich.
As well as securing these three new players, sporting director Michael Zorc also decided to pull the trigger on Paco Alcacer and pay the €23m (£20m/$26.5m) purchase clause in his loan agreement with Barcelona.
The Spanish striker scored 18 goals in 26 Bundesliga games last season, but is still not a guaranteed starter as Favre sees him as the ideal impact player off the bench against tired legs. At Barcelona, he probably would have received even less playing time, but important figures inside the Catalan club are disappointed by his departure.
Barcelona's loss is Dortmund's gain, and chief executive Hans-Joachim Watzke and sporting director Zorc have no intentions of closing the chequebook just yet.
Another striker could come in this summer to further boost their chances of returning to the very top of German football.
Bayern Munich are often accused of 'buying the Bundesliga', but a decade and a half after they gave Dortmund a loan to secure their existence, BVB are showing they are not shy to splash the cash, too.