From: Rabbi Akiva Eiger
To: Unknown recipient
Rabbi Akiva Eiger rules that a borrower may deduct interest collected by an orphans' guardian from the principal still owed, using a mimenei nafshach argument showing that under virtually every combination of the relevant opinions regarding dichui and gozel u'ma'achil the borrower has valid grounds for the deduction. He also rules that a claimant may entirely forego a shevuas hashutfin and instead demand only a shevuas heses, treating himself as an ordinary claimant.
Source: Shu"t Rabbi Akiva Eiger §54