At €28 million, Borussia Dortmund's purchase of Abdou Diallo (right) from Mainz is the most expensive made by a German club this season. Photo: EPA/ARMANDO BABANI

DUSSELDORF, Germany - Champions Bayern Munich have not spent a cent on new players while rivals Borussia Dortmund lead the way with 73 million euros (84 million dollars) invested in new personnel ahead of the Bundesliga season starting on Friday.

Bayern's restraint is among the reasons why the 18 clubs' investment of so far 413 million euros is far below the 600 million euros spent a year ago by the top flight sides, according to a dpa survey.

The transfer window however remains open until August 31 amid speculation that Bayern could sell defender Jerome Boateng and midfielder Sebastian Rudy. In addition, Dortmund still lack a true striker.

Bundesliga clubs have so far managed a surplus of 10 million euros with transfer income of 423 million euros. Leipzig raked in 70 million alone for selling Naby Keita to Liverpool while Dortmund were the biggest spenders in their big to get closer in the standings to Bayern again.

Dortmund have three of the four most expensive newcomers in the league: Abdou Diallo (Mainz/28 million euros), Thomas Delaney (Bremen/20) and Belgian Axel Witsel (Quanjian/20).

Whether they also get a big-name striker in succession of Pierre-Emerick Aubameyang remains to be seen as sports director Michael Zorc named sums being paid for top marksmen "unrealistic" for his club.

Three others had club record signings as Borussia Moenchengladbach spent 23 million euros for Nice's Alassane Plea, Werder Bremen 15 millionen for Everton forward Davy Klaassen and Mainz 8 million for Olympique Lyon's Jean-Philippe Mateta. Munich meanwhile had Serge Gnabry and Renato Sanches return from loan speels, and Germany midfielder Leon Goretzka arrived from Schalke on a free transfer.

Instead, they became even wealthier, with Douglas Costa sold to Juventus for good for 40 million euros and Arturo Vidal's sale to Barcelona generating 18 million. Munich want 50 million for Boateng and at least 15 million for Rudy.

All this could be used next year when they are expected to splash out 42 million euros on a permanent move of midfielder James Rodriguez and 35 million on VfB Stuttgart's France defender Benjamin Pavard. "We are collecting money should be have go shopping a little bigger next year," club president Uli Hoeness said.

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