Nedarim 59: Davar Sh'Yesh Lo Matirin

If a person vows that certain vegetables are konam, s/he is forbidden to benefit from any earnings through the trade of that vegetable and anything that grows from that vegetable.  However, if the vow is simple and non-specific, s/he will be permitted to benefit from anything that grows from that vegetable, as long as the seed/bulb eventually rots.  

A principle is stated: konamot are considered to be "davar sh'yesh lo matirin", an item that can become permitted with the permission of a rabbi.  Teruma does not fall into this category.  The rabbis discuss what should be done if an item is both konam, and can become permitted, and teruma, which cannot become permitted.

In this discussion, the rabbis also consider the role of timing in making and keeping vows.  Once an action has been completed regarding one's vow, the vow cannot be revoked.  And thus if one gives teruma to a priest, the vow that allowed the produce to be consecrated cannot be undone.

The rabbis return to the onion discussed in yesterday's daf.  If an onion bulb planted in the sixth year and it was rained on until its leaves turned black, it is forbidden.   Rabbi Shimon ben Gamliel teaches that the part of the onion that grew during the sixth year is permitted while the part that grew during shemita is forbidden.  They go on to explain other circumstances that growth of a plant determines whether or not it is permitted.