now we have a link to a certain rich, young man in the NT, whose face was also downcast by the reception of what he 'offered' ((re: Matthew 19:22, Mark 10:22)) -- if Cain really had not brought the firstfruits, or the 'best' of the crop he had raised, then we also have a typological correlation with how this man had given his 'efforts' to God ((in that he had kept the Law from his youth, at least according to his own estimation)), but in a sense his heart was given to his earthly wealth.
the connection of wealth brings us back to Balaam ((re: Jude 1:11)) and also to Korah if we conflate riches with the power/leadership position that he was seeking to usurp from Moses ((Numbers 16)). even though Balaam was wroth to pronounce anything God had not spoken, he was willing to seek out a way to bring a curse on what God blessed, for the sake of lucre. comparatively, Cain, if he indeed kept back the best/first of the fruit of his labor in the fields for himself, would have been exhibiting covetousness in a similar way, a sin which eventually expressed itself in murder.