According to Spanish information, the Premier League club must pay close to SEK 750 million for the Swedish national team star, which is slightly more expensive than Zlatan Ibrahimovic's transfer to Barcelona in 2009 (SEK 735 million).
The transfer fee means that Alexander Isak becomes Newcastle's most expensive signing ever, and likewise Real Sociedad's most expensive sale.
Three years in La Liga
One of the reasons why the deal can be made possible is the Saudi Arabian investment fund PIF, which last autumn bought 80 percent of the shares in Newcastle, which has given the club greater financial muscle.
Isak came to the Spanish club three years ago and made a big success the following season when he accounted for 17 league goals.
After that, it has been all the more difficult with the striker's goal production.
In Newcastle, he is joined by national team colleague Emil Krafth.
From the archive: Alexander Isak's dream debut in Allsvenskan 2016
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From the archive: Alexander Isak's dream debut in Allsvenskan 2016