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Question: Which beis din can declare items Hefker?
Answer: Encyclopedia Talmudit discusses this question, at the entry "הפקר בית דין," section ד.

Some ראשונים hold1, based on the gemara Gittin 36b, that הפקר בית דין can only be done with a בית דין of the caliber of that of רב אמי and רב אסי, and hence it cannot be done today.  

Others posit that the comparison to רב אמי ורב אסי is not literal, but is really meant to be taken in the sense of "the biggest בית דין of the generation,"2 or whichever בית דין in town that has the ability to be מוציא ממון, to enforce the payment of money.3  

There is also an opinion out there that any בית דין is capable of hefker beis din4, and when the gemara mentioned רב אמי ורב אסי, that was just to exclude a בית דין of הדיוטות, common people (i.e., without semicha.  

However, there is an opinion that even a בית דין של הדיוטות has the power of הפקר בית דין, in line with the supposed statement by Rav Schachter.5



1 רבינו תם in ספר הישר, siman 270; רא"ש גיטין, perek ד number 13; בית יוסף CM 67; מרדכי גיטין perek ד number 381; and more.  

2 רבינו תם in ספר הישר, ibid.; תוספות to Gittin 36b; רא"ש and מרדכי, ibid.; סמ"ג, Negative Commandments 271; סמ"ק siman 258; and more.

3 ספר הישר and ראבי"ה.

4 שלחן ערוך.

5 ריטב"א Gittin 36b; שו"ת רדב"ז, and רדב"ז על הרמב"ם.