(R. Yaakov bar Acha/ Abba bar Chiya citing Rav): (Proving that it's prohibited to intentionally mix in a second species and then sell the mixture as first species.) One who selects out pebbles from his friend's stack must replace them with fine wheat stalks. (When selling produce, it may contain a small amount of impurities, so by taking away the pebbles, he has caused loss to the seller, who must now fill that a mount with actual produce. Therefore, he must reimburse the seller.)

(R. Yosi): (Why not just mix the pebbles back into it?) This shows that one may not intentionally mix in another species (or pebbles) as if it would be permitted, why would he need to replace them with wheat stalks?!

If he sold him fine wheat and it's found to be without any impurities, may the seller subtract from it the amount that is permitted for impurities? From the words of R. Yaakov bar Acha earlier, it appears that he may not subtract, as he said, "One who selects out pebbles from his friend's stack must replace them with fine wheat stalks''. (If the seller could subtract, why must the person replace what he selected?)

Question: If he selected out one and a half quarter Kavs, may the seller subtract the amount of one quarter (which is the amount permitted for impurities)?

Answer: R. Chinana and R. Mana disagreed - R. Chinana said that he may subtract it; R. Mana said that he may not, as the seller could say him - if you wouldn't have selected and I would have had the mixture, I would have wrapped all of it in a sack and told the buyer that there are more impurities than a quarter Kav - and sometimes he wouldn't mind. But now that you've selected it, I cannot return the impurities to it so you should pay me for that. (However, we see from this that one is forbidden to mix anything else into it...?)

Answer: (He didn't intentionally mix them.) When he gathered the produce, the other seeds became mixed in.

Question: Didn't the Mishnah teach (above Menachos 30(f)) that the Halacha is that garden seeds that are not eaten (such as garlic or turnip seeds) combine to be 1/24th in a Beis Seah...

Baraisa: (When does this apply?) For example, a Kav or half a Kav of garden seeds per Beis Seah will suffice to prohibit the mixture - but is 1/24th of half a Kav good for anything?

Answer: Until this limit of 1/24th, they were concerned for Maaris HaAyin (that he appears to be planting Kilayim); but beyond this (i.e. less than 1/24th), they were not concerned.