I believe the price at the Embassy already reflects a 20% discount. Not a Ferengi but I do have the Marauder Set power which also grants 20% discount on anything I but at Sector Space, which has more selections than the discounted commodity vendor at the Embassy and the prices are the same. Effectively, if the Ferengi discount applies, it would be another 20% on top another 20%. So the discount would be greater than 20% and that wouldn't make sense.
As I wrote here, it appears that the Ferengi can get 20% off the embassy commodities console and still be at the safety of 60% of the base price -- which would not fall below the infinite-money 50% mark.