Colin Calderwood's move to Aston Villa as assistant manager from Brighton was confirmed by Seagulls boss Chris Hughton after Friday's 1-1 draw between the two Championship sides.
Ex-Northampton and Nottingham Forest boss Calderwood, 51, is a former Villa player and lives in Northampton - meaning a shorter commute.
Calderwood had quit before the match.
"It has been agreed for Colin to go to Aston Villa and he is free to go as of this moment," Hughton told BBC Sussex.
"I'm disappointed and very surprised."
In a club statement realised earlier in the day, Brighton said Calderwood, who was placed on gardening leave on 12 November, had left the club for a job "elsewhere".