Several outlets had already reported an agreement in principle for the transfer, but the deal has hit serious trouble in recent days, with Leipzig said not to have even responded to Madrid's latest written approach.
Even so, it remains entirely possible that Diomandé, who was named the Bundesliga's Rookie of the Season last term after scoring 12 league goals, eventually completes the move to the Spanish capital; some reports value the deal as high as €135m once bonuses are included.
RB Leipzig are doing their homework behind the scenes in the meantime, searching for candidates to replace the 19-year-old right winger, mindful that his exit would leave a significant creative gap in Ole Werner's attack.
According to Foot Mercato, Leipzig's recruitment staff have identified Chemsdine Talbi as one option, with the versatile forward rated highly within the club's hierarchy for his speed, ball-carrying and flexibility across the front line.
The Morocco international, who switched allegiance from Belgium, caught the eye at this summer's World Cup, featuring in five of his country's six matches and playing more minutes as the tournament progressed, including 85 minutes in the quarter-final defeat to France.
There has been no contact yet between the Bundesliga club and Sunderland, where the 21-year-old right-footer scored four goals in 30 appearances last season and is under contract until 2030 having joined from Club Brugge in the summer of 2025.
As further candidates for the role, Leipzig are also weighing up Francisco Conceição (23, Juventus), Paul Nebel (23, Mainz), Malick Fofana (21, Lyon), Tyler Dibling (20, Everton), Ethan Nwaneri (19, Arsenal) and Said El Mala (19, Köln), though none of those alternatives have progressed beyond internal discussions at this stage either.
RB Leipzig's approach underlines a club determined not to be caught short regardless of how the Diomandé saga eventually resolves itself.