Confirm rates in writing at moment of agreement

As a landlord you agree a good rate with an agent. He promises to agree in writing. By the time you drive home you and your colleague can't agree on which rate you agreed. Two days later You are negotiating with two other agents. By this time you can't be sure what you agreed with the first agent, nor that he won't change his mind or forget.
   One solution is to text your colleague and copy the agent, or vice versa, immediately.
If there's any disagreement, they must get back to you at once.