If the breeder doesn't have a contract, look elsewhere. All the promises and guarantees mean nothing unless it is spelled out in writing. If there is something in the contract you don't understand, consult with a lawyer. The contract should state the following:
The dog or puppy's identification. Usually the puppy litter number, a name, or some other distinguishing feature, including the puppy's parents.
The guarantee that the puppy is healthy and free from genetic diseases. The breeder usually has a reasonable time limit on the guarantee and usually offers another puppy in exchange or a refund.
Right of first refusal (or first right of refusal). The breeder stipulates that if the owner can no longer keep the dog, at any time in the dog's life and regardless of reason, the dog be returned to the breeder.
A guarantee that the breeder will take back the dog under any circumstance.
That you will adequately care for the puppy. This includes adequate food, water, shelter, training, socialization, and health care.
Why should you care about what's in a contract? Besides the fact that the contract is your bill of sale, it is also the proof that you bought the dog and gives you some legal recourse in case something happens. For example, let's say that your rottweiler has hip dysplasia. The contract should state what the breeder will do if your rottweiler is proven to have hip dysplasia.
The right of first refusal or first right of refusal gives the breeder the right to take the dog back if you no longer want him. However, that alone doesn't protect you, the buyer. There has to be a clause in the contract stating that the breeder will take back the dog, regardless of circumstance. This puts the breeder on the hook for placing the dog if there's a problem. A reputable breeder will do this but will also screen potential puppy buyers because she doesn't want a dog to come back.
Unscrupulous breeders may not honor contracts. If this is the case, you may have legal recourse. As it is expensive, most breeders aren't willing to put up with a potential lawsuit.
What's also very important are any clauses in the contract requiring special conditions, such as co-ownership or stud rights. Unless you've agreed to these types of conditions beforehand, it is unwise that you sign a contract limiting your ownership of your dog. Likewise, avoid buying from anyone who has statements that require special diets or conditions that can't be reasonably met. These conditions often void the breeder's agreement to replace or refund a dog that has a genetic disorder.
Lastly, you should receive the AKC puppy papers (with the box checked for limited registration if your rottweiler is being sold as pet quality). If the breeder doesn't furnish your rottweiler's puppy papers, then you should find another dog. There's no reason the breeder shouldn't provide your rottweiler's AKC papers, unless your prospective rottweiler isn't registered. In this case, the breeder is charging you purebred prices for a dog that may or may not be purebred. He or she may be breeding unregistered dogs that were never supposed to be bred. If you've gotten this far with seeing OFAs, CERFs, AKC registrations, and the like, it is extremely unlikely that this would happen, but be on the lookout.