I imagine that the conversation went something like this:
IGMUND: Ulfric! Can you help me take my city back from those barbarians?
ULFRIC: Possibibly, but you will have to make it worth my while.
IGMUND: What would make it worth your while?
ULFRIC: I want Freedom of Worship for all Nords in Markarth and the Reach.
IGMUND: Anything! Whatever you want! Just get me my city back!
What is missing and should have been addressed by the Empire is that after the Markarth Incident where the Empire forced the agreement to be nullified is that Igmund should have been arrested and sent to Imperial City to be tried for high treason. For having knowingly and willfully violated an Imperial treaty and thereby deliberately risked the collapse of the tenuous peace secured by the WGC.