Q. If I buy more chometz after making the Rav my agent to sell my chometz, will the new chometz be included in my sale or is it a davar shelo ba l’olam regarding my original agreement with the Rav?

Q. (Similar)- If I buy a bottle of whiskey after I make the Rav my shaliach to sell my chometz, will the whiskey also be sold to the Goy? Will it be assur after Pesach?

A. Nitey Gavriel (Pesach 1: 40: 13) quotes Bigdei Yesha (4) and other Poskim that on the onset one cannot name an agent to effect on something that is not yet in his possession. Poskim recommend, to contact the Rabbi who is acting as his agent and inform him so he will include the new acquired chometz items in the sale. (See also Mechiras Chometz Kehilchoso p.21.) After the fact the chometz is permitted.
Horav Shlomo Miller’s Shlit”a opinion is similar.

Rabbi A. Bartfeld as advised by Horav Shlomo Miller Shlit”a