From: Rabbi Moshe Sofer
To: Rabbi Yisrael
When a 100-zehuvim banknote was stolen after a currency exchange, the Chasam Sofer ruled that Shimon incurred no guardianship obligation since placing a note on a table does not effect a kinyan; liability turns on whether the note was placed before or after receiving the small notes, with doubt requiring payment. He further ruled that banknotes have the status of currency by dina d'malchusa, valid for pidyon hekdesh and oaths, but not for pidyon haben, which requires actual silver weight since the obligation is to God directly and earthly royal decrees are irrelevant to Him.
Source: Shu"t Chasam Sofer CM §187