In Misty of Chincoteague, what did the men need to do in order to fasten a sold rope around a ponies neck?

They needed to earn at least one-hundred dollars before Pony Penning Day, or they would have to remove the rope, and she would be open to other buyers.

"No, I'll tie a rope around her neck to show she's already sold. To me. To us," he added hastily, thinking of the cost of her. "She'll sell for around a hundred dollars^ maybe."