As Rico mentioned earlier the trade rules we have always used in this game are Offer, then Acceptance, then Confirmation.
So, if I offer a deal to another GM, then the other GM agrees to the deal, I still have to confirm the deal before it becomes official.
If I choose to back out, even after the other GM accepts an offer that I made, that is allowed. Though I would say that in most circumstances it's very bad trading etiquette and I would be pissed if someone did it to me.
There are a couple of reasons why this is the rule that we follow. One being that if I were to make an offer to another GM and they respond a month later, that should not be official.
The other reason is that many people will make offers that are dependent on other circumstances
(whether they have traded left in the month, whether they can move a different player
etc.), in these circumstances it is always good trading etiquette to mention what the offer that you are making is dependent on.
If you guys need any more help to resolve this issue feel free to message me on Twitter