Unai Emery is keen to have Diaby at Villa Park and the Premier League side are working hard to try and deliver for their Spanish boss.
They face competition from wealthy Saudi side Al Nassr, but now Aston Villa have a breakthrough in place.
Villa have an agreement in principle with Leverkusen, according to Sky Deutschland.
The deal will see Aston Villa pay around €50m, plus add-ons, for the France international winger.
Al Nassr have not given up hope of winning the race and are still pushing to snap up Diaby, but Aston Villa are now feeling more confident.
It is suggested that it is the Premier League side that are making the running in the race for Diaby.
The decision may end up coming down to the winger and where he sees his future, with the Premier League and the Saudi Pro League his potential destinations this summer.