The Bundesliga club have reportedly paid an initial fee of £4.5million which could rise a further £800,000 if they qualify for Europe.
Hamburg were believed to be in the running for the talented 23-year-old in January, but saw a £3.2million offer rejected.
On Thursday Hertha head coach Markus Babbel insisted Kacar was part of their plans but that a 'lucrative offer' could change the club's mind.
Neither club have yet to comment on the report. Kacar is contracted to Hertha until the summer of 2012.