The Argentine is set to play for the Reds next season after an impressive campaign.
The World Cup winner has signed for a modest £35 million, which is significantly lower than most pundits had predicted.
Fabrizio Romano has outlined the fee, stating that Mac Allister had a release clause in his contract.
Most had been expecting Mac Allister to leave for a fee in the region of £60 million.
Brighton may also lose Moises Caicedo this summer, but he does not have such a release clause in his deal.