This seems to be a difficult verse to translate, as every one I have renders it differently.
Somebody may have just the answer you're looking for, but in the meantime remember that no scripture is it's own interpretation. When I encounter these difficult sayings, I stick them in my brain and hold them there for a while until I find another witness in Scripture and Spiritual match. Until then, for me, it's just something to think about.
Anyways, a similar and maybe related question has been nagging me lately. This doesn't really clarify it at present, so I'm going to have to leave that one alone,
What I think is that The Lord's question is about the state of Cain's anger and countenance (his reaction) to the passing over of his sacrifice.
Here's the LITV rendering:
Gen 4:5 And He did not look to Cain and to his offering. And Cain glowed greatly with anger, and his face fell.
Gen 4:6 And Jehovah said to Cain, Why have you angrily glowed, and why has your face fallen?
Gen 4:7 If you do well, is there not exaltation? And if you do not do well, sin is crouching at the door; and its desire is toward you; but you should rule over it.
Cain, your are angry and your face is fallen. If you do well, isn't the opposite true--exhaltation instead of anger and a fallen countenance (Shame? Depression?)
If you don't do well...And I guess that's where your question comes in. Maybe it's a bit less doctrinal and a bit more practical. Doing well brings consequences. Doing not well brings it's consequences too, in anger, shame, depression, cynicism. I've known quite a bit about that.
Still, it may really SHOULD be rendered 'sin-offering'. It's harder for me to make sense of it that way, but that doesn't mean it's 'wrong'. Another one to stick in my brain, maybe.
Maybe that helps...maybe it doesn't muddle too much.