€60m release clause, and they seem genuinely unwilling to entertain offers much below that. They appear to be more building team around him, than expecting or planning for his departure.
When he left Wolfsburg, they wanted to keep him, but his contract was running down and he wanted to play more. Newcastle were in for him last summer and he chose Sporting over them, they have bid again this summer, but seemed to drop out and said they didn't want to get into a bidding war.
If a team was interested to the point of paying the release clause, then they can somewhat chose when, as Sporting have only influence, not authority if release clause is met.