Hey, 

Not entirely confident this is a bug, perhaps it is an intentional feature. 
I had a trade set by someone with me today - they added 8 items and accepted before I could add money. This locked the ability for me to add items or money to the trade without cancelling it. 

Hey PASQI,

That is intentional, once a player clicks Accept the trade can't be modified any longer, this was intended to stop players potentially being able to quickly modify trades after a player thought it was set.

yeah you have to start a whole new trade.