that neo-soul was likely born with "Brown Sugar" and named with "Baduizm" though the other suggestions are valid (right and wrong are subjective). I think the topic being raised here by the OP is the gestation period for neo-soul.

With that being said, I would like to suggest Soul II Soul's "Keep on Movin'" as a notable precursor to the neo soul period. Soulful melodies and harmonies combined with a hip-hop attitude towards drumming and sampling are what I consider hallmarks of the neo soul sound. I can't think of anything (enlighten me if necessary) on a popular level that was released during that time (1989) other than "Keep on Movin'" that fully embraced the smooth, soulful, jazzy and afro-centric musical sound with a hip-hop vibe that would later on be recognized as neo-soul.

The grooves on "Keep on Movin'" may not have the relaxed quantization of the post-D'Angelo period and the oft-used Rhodes sound isn't prominent but Caron Wheeler's image as a modern afro-centric female vocalist predates Erykah Badu by almost a decade.

I could go out on a limb an suggest that Guy's debut was an early neo-soul recording as they also combined R&B and hip-hop in their production. Also, remove the vocals to Kwame's "The Rhythm" and you'd have a credible neo-soul track, Rhodes and all, even by modern standards. After Soul II Soul, I think the arrival of A Tribe Called Quest (Bonita Applebum) accelerated the move to what we hear on "Hey DJ", "Brown Sugar" and "Baduizm" among others.

I could get really adventurous and declare that the original neo-soul album was "Diamond Life"...LOL! However, Sade's popularity predates the national popularity of hip-hop.