The type of rule you are asking for is similar to the Chess rule that says...
Once the hand is taken off a piece after moving it, the move cannot be retracted (unless the move is illegal)."
Catan rules aren't that specific, unfortunately. There is no rule in Catan that covers when a trade is complete with this level of detail. Therefore, it is up to you and your play group. If I were in your play group, I would argue that a trade is complete when the cards have physically changed hands. That is how goods exchanges work in real life - I give you money, you give me goods.
If you allow trades to be taken back after cards have exchanged hands, then you have to remember what cards you traded. More importantly, you have to still have the cards, and not have spent them.
If you don't allow trades to be taken back before cards have exchanged hands, then what happens if I don't have the cards I said I would trade? Maybe I wanted to bluff that I had a card for trade just so the other guy would get a worse deal.
Bluffing is generally an acceptable practice in any game that involves hidden information. But, the difference between lying and bluffing is not always clear. In Catan, you just have to know it when you see it. For example, saying "I have wheat." when I really don't is a bluff by any reasonable definition of the word, but saying "I have seven cards." when I really have eight is just a lie. If your group is not okay with bluffing, then your play group may need to create a house rule that prohibits it. Otherwise, it's fair game.