Carney Chukwuemeka goes to Borussia Dortmund in a deal that will take Chelsea's summer turnover over the £ 270 million.
Winger Chukwuemeka, 21, spent the second half of last season on loan at Dortmund and will permanently sign for £ 24 million.
The Bundesliga-outfit has also agreed a seasonal loan agreement for another spare part of Stamford Bridge, defender Aaron Anselmino.
Chelsea also wants to sell Nicolas Jackson and Christopher Nunku forward before the transfer window closes.
Teen Tyrique George could also leave – and Chelsea is desperately to be about high earners Raheem Sterling, Ben Chilwell and Axel Disasi.
England Under 20 wing player Chukwuemeka was also a member of the 'Bomp team' executives from first team training until their futures have been resolved.
The blues spent £ 277 million on new faces this summer – but it has been told that they have to beat that figure on sale to register them for the Champions League.
UEFA has imposed a fine of the blues £ 27 million in July for rules for the Increase Team Cost ratio.
The Chukwuemeka -deal takes their total turnover at £ 273.4 million.
Baas Enzo Maresca wants to add Manchester United Winger Alejandro Garnacho and RB Leipzig -Midfielder Xavi Simons to his swimming pool this month.
The Chukwuemeka deal includes add-ons and a sales clause, but Dortmund has no option to buy Anselmino.
The Argentinian, 20, has only played once since he came to Boca Juniors for £ 15.6 million in January, when he got off the bank against Benfica in the Benfica in Extra Time in the World Cup club last 16.
Chelsea boss Maresca also wants to sign a center-back to replace the wounded Levi Colwill.
But club heads would like to find an internal option, with Tosin Adarabioyo, Wesley Fofana, Josh Acheampong and Benoit Badiashile possible partners for Trevoh Chalobah.