Borussia Dortmund are planning to test Mainz's no-sale stance on Arsenal target Abdou Diallo.
The defender impressed in the Bundesliga this season and Mainz know he is attracting serious attention from a number of clubs.
Arsenal have been linked with Diallo, but Mainz have insisted they have no intention of selling a player they only signed last year from Monaco.
But according to German daily Bild, Dortmund are ready to test Mainz's resolve by slapping in a bid for Diallo.
The Ruhr giants believe they may lose Sokratis Papastathopoulos to Arsenal after he refused to extend his contract.
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Dortmund expect to receive €20m if they sell Sokratis and are prepared to put the money into a swoop for Diallo.
Mainz paid Monaco a fee of €5m to sign Diallo last summer and tied him down to a five-year contract at the club.
It remains to be seen whether Mainz can resist making a quick profit on the defender.
