His agent is ICM Stellar Sports, one of the biggest in the business, they represent a lot of the England players (Grealish, Phillips. Chilwell, Shaw, Ward-Prowse, Pickford, Loftus-Cheek etc).
And this is where it gets sticky for the club, because these agents have access to a lot of clubs that wait in the wings for players they recommend to them, Boro will not be the only club in the wings some will be biding their time to see developments before coming out of the woodwork to show an interest plus the agents will probably already know which clubs are interested in greaves and now greaves will also have been informed of this hence the stand off.
These are top agents so if they recommend Greaves to a club they will take notice.