What is a Sixth?

Shmuel states that ona’ah includes a case of a sixth of the sale price, even if it is not a sixth of the fair price. Shmuel also agrees that a sixth of the fair price is considered ona’ah. Therefore, according to Shmuel, all the cases below are ona’ah:
1. Item worth 70, sold for 60
2. Item worth 60, sold for 50
3. Item worth 60, sold for 70
4. Item worth 50, sold for 60

The Rambam (Mechira 12:3) says that anything less than a sixth is considered forgiven. The Rambam gives two examples:
1. Item worth 60, sold for 51
2. Item worth 60, sold for 69

The Magid Mishnah challenges the first case of the Rambam. In this case, a sixth of the fair price is 10, while a sixth of the sale price is 8.5. The difference in the sale is less than a sixth of the fair price, but is more than a sixth of the sale price (8.5), and should be ona’ah according to Shmuel!?

The Shulchan Aruch (HM 227:3) only discusses the second case, while the Rama (227:4) cites the first case.

The Sma (227:5,11) rules like the Magid Mishnah.

The Taz explains that the Rambam holds that Shmuel accepts either form of sixth, but only at a sixth. Any other deviation is evaluated based on the fair price only. See Drisha C”M 227:4 for more details.