Daf 10b has a discussion of a hidden blemish that upon discovery between kiddushin and nisuin could create a "simpon" (annulling force) to retroactively cancel kiddushin.
Then on 11a, R' Yochanan ben Bag Bag says that there is no such "simpon" that could annul the purchase of a Canaanite slave. The Gemara explains that exposed defects would be known to the buyer before purchase, while hidden defects shouldn't be of any concern to him.
I imagine that we are only talking about something considered to be superficial, such as a physical blemish? If something more serious is discovered, such as the fact that the slave is an aylonis, would that annul the purchase, or not?
Even if only a superficial physical blemish is being addressed, what if the owner wants to pair a female slave with another slave to obtain offspring, and it is made difficult or impossible because of the emotional effect this hidden blemish will have on the potential mate?
The BEIS HA'LEVI (3:3) points out that although hidden blemishes on the man will also annul Kidushin, still we are not concerned that maybe the woman will find such blemishes and annul the Kidushin. He deduces from this fact that not every time that there is a possibility of Mekach Ta'us were the Chachamim concerned about Simpon. Surely by Avadim there are also such hidden blemishes, but nevertheless since it is not generally the case, the Chachamim allowed him to eat Terumah. The Beis ha'Levi writes that since "Ein Ishah Ela l'Yofi," it is more likely that there will be a hidden blemish that will annul the Kidushin. Even in your case of the Eved, it is unlikely that that is the whole reason for the purchase of the Shifchah nor is it generally the case and therefore there was no Gezeirah of Simpon by Avadim.
D. Zupnik