The former Barcelona academy player moved to PSV in June 2022 from PSG upon the expiry of his contract, before rejoining the French club, who have immediately loaned him to Leipzig.
Dortmund, who travel to Chelsea on Tuesday for their Champions League Round of 16 second leg leading 1-0, have improved their club record with their best start of the year, even better than in their 2011/12 title-winning season.
Leipzig are in fourth place on 36 points, three behind visitors Union, who have won more league matches this season than leaders Bayern Munich.
RB Leipzig named Marco Rose as new head coach on Thursday a day after sacking Domenico Tedesco.
Hertha dropped Cunha from the matchday squad for Saturday's 2-1 home league defeat to Wolfsburg while the transfer was finalised.
Haaland has scored 37 goals in 38 games for Dortmund this season in all competitions.
Salah scored the opening goal against RB Leipzig to help his team secure a 2-0 win and put one foot in the prestigious competition’s quarter-finals.
RB Leipzig were only promoted to the Bundesliga in 2016 and they continue to break new ground with the last 16 win over Tottenham Hotspur.
RB Leipzig have given Timo Werner an ultimatum to sign a contract extension by the end of the season with Bayern Munich already having expressed interest in the Germany striker.
Liverpool reached an agreement with RB Leipzig and Keita, who will play for the Reds next summer.
Turkish champions Besiktas reached the last 16 of the Champions League for the first time as they fought back for a 1-1 draw.
Last year's semi-finalists Monaco crashed out of the Champions League in the group stages when they were beaten at home 4-1 by Leipzig on Tuesday.
Bayern Munich succeeded in securing an important victory over RB Leipzig (5-4) with penalties after they ended the full time with a draw (1-1).
Guinea international midfielder Naby Keita will join Premier League giants Liverpool next year after the English outfit agreed a reported club record deal of £48 million.
The UEFA announced on June 20 that FC Salzburg and RB Leipzig will be able to play Champions League football next season.
The French talent Ibrahima Konate will officially be joining RB Leipzig in July.