How legally take adopted out dog back?

We are small dog rescue, rescued 5 month old puppy and adopted out, our vet missed that the pup was pregnant, we have 30 days waiting period for adopter to return dog back if it does not work, our vet missed that the dog was pregnant as she was very skinny and scared, she supposed to be spayed within a month as we do spay /neuter at age of 6 month, but adopters did not tell us that she was pregnant till she was due, when they notified us we went to the house and confirm that the dog was too far for terminating pregnancy, new adopters volunteered to foster care for pups till they are at age of 2 months to be adopted out, now they refused to let us vaccinate, microchip and neuter/spay claiming pups are their and they are going to keep them or/and give to their relatives, when dog was adopted out they signed documents that as rescue we have rights to take dog back if we suspect any unappropriated business, which we are now, how can we proceed to remote pups and female? Thank you, Tiana