Date: 1804
From: Rabbi Moshe Sofer
To: Rabbi Meir
The Chasam Sofer rules that the majority of poskim hold Shemittah cancels debts only at the end of the seventh year, not at its entry, so there is no strict requirement to write a pruzbul during the sixth year. Nevertheless, if one wishes to be stringent and write it early, a predated pruzbul is valid and there is no concern of yuhara. He also addresses subsidiary questions regarding rented land and transfer of lien rights for pruzbul purposes.
Source: Shu"t Chasam Sofer CM §50