The Brazilian-born Spain striker made three requests to leave Chelsea during the course of last season. After asking to join Atletico Madrid last summer, his intended destination in January was Tianjin Quanjian, of the Chinese Super League, before he reset his sights on Spain with a third request, which was granted towards the end of the campaign, leaving only the terms of his departure to be finalised this summer.
https://www.thetimes.co.uk/edition/sport/diego-costa-asked-chelsea-to-sell-him-during-last-summers-pre-season-tour-h07wmdcdv