Did your adoption group tell you you were adopting a high prey greyhound? It's not the greyhounds fault. He should not be around small dogs. Use a muzzle before you end up paying someone's vet bill or your greyhound gets put down. If an owner gets between the small dog and your greyhound and blood is drawn he can be labeled as a vicious dog. Once he is declared a vicious dog he will be put down.
Unfortunately this happens.
For your safety and the greyhounds safety it would be best to return him. They might have a return that could care less about small dogs. Your adoption group should have advised you better.