I, as a non-native speaker, used the sentence "I'm not sure if I agree." thinking it means that I have a level of uncertainty about where I stand, and apart from that I can lean anywhere, I can even be very close to agreeing. I was told that this expression actually means "I disagree". So which one is it? Is the meaning literal or is it more on the idiom side? What would you think I'm leaning towards when I say that sentence?
In the U.S., it could mean you are trying to gently say "I disagree." You might say it this way when being argumentative can get you in trouble or sour a relationship.
But it could also mean you want to explore additional facts and/or reasoning before agreeing.
Suppose you said "I'm not sure if I agree. Have we checked all of the regional sales reports?" Then it would be clear you are seeking additional information before agreeing.
If you said "I'm not sure I agree." without anything after it, it could mean either of the two cases. The context of the conversation would help determine which case it is.