The server rules do not enforce a payback of excess money given by accident that is not covered by an agreement of any kind. However, refusing to pay back money one should have never received can permanently damage one's reputation, which isn't really worth it.
If you make an approved contract with the person that specifies the exact amount that the user should pay you, then yes as not returning excess payment could be considered a breach of contract.
If a contract exists, the predefined amount is covered by the contract, but anything additionally sent should not be protected by the contract.
True, though it generally varies from contract to contract. Some contracts, usually for construction projects, contain sub-clauses that say that excess payment should be returned. All the more reason to be careful in your transactions ultimately.
