They had £30M on the table from Everton.
http://www.standard.co.uk/sport/foo...-mauricio-pochettino-wanted-the-a3334451.html
http://www.tottenhamblog.com/2016/09/01/how-much-are-spurs-really-paying-for-moussa-sissoko/
I guess no one knows the full details but this is what I read.