what labor did Abel do to cause his livestock to mate & give birth?
leave a flock of sheep alone and they're going to produce little sheep without your intervention.
leave a garden alone and it's going to become a bed of weeds soon enough.
i really think that any argument about 'works' of one of the brothers hands really goes the opposite of what you've put, Newton. Genesis 3:17-19 describes strenuous toil in relation to cultivating foodcrops, and Genesis 4:2 specifies Abel 'kept' or 'tended' flocks while Cain 'worked' or 'tilled' the ground. that's pragmatically overseeing/managing on Abel's part, and physical labor on Cain's.
i should say, i have heard this description in sermons before - the idea that Cain lazily yanked up some wild grapes or something on his way to the altar at the last minute. it makes for a good topical example if you intend to preach at your congregation, chiding them for slothfulness or something, but it's a narrative that's not actually in the Bible.