Leave the toy in the dog's mouth and create space. A growl around a valued item is a bite warning, not a sharing lesson. Ask other owners to keep dogs away. Call your dog toward a quieter area only if they can move willingly and safely with the item. Do not chase, grab their collar or put a hand in their mouth. If you cannot safely recover someone else's toy, tell its owner and ask for time and space rather than provoking a bite. Do not leave the situation unresolved or let the dog re-enter a crowd with the toy.
Pause toy-filled dog-park visits until you have a plan. At home, with no other dogs nearby, a qualified reward-based trainer can help teach a safe exchange and a cue to leave a toy. The article on whether the dog park suits this dog offers alternatives for exercise and social time. The goal is not to prove they can share at a busy park.
If there has been a bite or fight, seek medical or veterinary care as needed and professional behavior support. Keep toys and high-value items separate from other dogs in the meantime. Dogs Trust and VCA advise management first for guarding, because repeated attempts to take an item can make the risk worse.
Today, change the park plan and keep your hands out of a guarded mouth. Safe distance protects your dog, the other dogs and their people.
Sources: Dogs Trust on resource guarding and safety, VCA on dog-park etiquette and risky toys, VCA on possessive aggression.