If you try clicking the trade under the "active" menu it wont let you cancel it if 20 turns havent passed?
maybe you can try a less conventional and a more random way to cancel it... say you're trading your only source of saltpeter to germany and you get into a war where you suddenly need it... just sent a unit to the saltpeter and pillage it
this will force the trade to cancel. if you are trading multiple sources though, you might have to pillage a few resources to cancle the specific trade you want... im pretty sure this wont cause a war if i remember correctly
and i think when they talk about the timer they are talking about the 20 turn limit