The Hammers have paid £45m for his services.
It was a condition from the player before he agreed to join the Hammers.
Haller is a quality striker who will improve West Ham a lot but the transfer clause in his contract leaves the Premier League club vulnerable to future offers from top clubs.
The Londoners could be under pressure to sell the player in future because of that agreement.
It will be interesting to see what happens in the coming years. The fans will certainly be worried about the clause.
It seems that Haller is using West Ham as a stepping stone right now and he is hoping to join a bigger club in future.
Having said that, if West Ham manage to fulfill his ambitions, he might end up staying for the long term.
The 25-year-old scored 15 goals and picked up 9 assists in the Bundesliga last season and it will be interesting to see how he adapts to English football now.