John owes Sally $100, he enters a contract to mow his neighbor Allen's lawn, over the next four weeks and have the Allen pay Sally $25 after every mowing. John notifies Sally by email that Allen will be paying her to satisfy his debt with Sally. Sally responds to the email by saying that is fine with me. When do Sally’s rights as a third party creditor become enforceable? Can John and Allen change the terms of the contract to only mowing the lawn every two weeks and if so do they have to tell Sally?