Talmud: The Mishnah teaches that an agent is considered to have diverged from the instructions of the homeowner if he gives each of the guests liver instead of meat or vice versa. This indicates that meat and liver are considered two different types of items, as giving one in place of the other would normally be done only after consultation with the homeowner. The Talmud asks: Who is the tanna who taught this halakha, that any case involving an item about which the agent would normally consult whether to give it or to give another item in its stead is considered to involve two separate items with regard to the prohibition of misuse?
אמר רב חסדא: דלא כרבי עקיבא, דתנן: הנודר מן הירק – מותר בדילועין, ורבי עקיבא אוסר.
Rav Ḥisda said that this is not in accordance with the opinion of R' Akiva. As we learned in a Mishnah (Nedarim 54a): In the case of one who takes a vow that vegetables are prohibited to him, without specifying which types of vegetable, he is permitted to eat gourds, as people do not typically include gourds in the category of vegetables; but R' Akiva deems it prohibited for him to eat gourds. R' Akiva maintains that as an agent would consult the homeowner before buying gourds instead of vegetables, they are in the same category. He reasons that if they were not both in the same category, the agent would not even bother consulting the homeowner if he would prefer gourds instead. Therefore, R' Akiva maintains that any item about which the agent would ask is included in the same category as the item he specified, and they are not two separate items.