Musar for Shabbat 296:10

ת"ש מסיפא אם לאו אינו משמט אי אמרת בשלמא ניתנה ליתבע היינו דקתני אינו משמט אלא אי אמרת לא ניתנה ליתבע אמאי אינו משמט דאי יהיב ליה שקיל

— In the first clause he must tell him, 'l release it,' while in the second he need not say, 'l release it'. As we learnt: If one repays a debt in the seventh year he [the creditor] must tell him, 'I release it;' but if he [the debtor] replies, '[I repay] even so,' he may accept it from him, for it is said, And this is the word<span class="x" onmousemove="('comment',' E. V. 'manner'. ');">¹⁵</span> of the release.<span class="x" onmousemove="('comment',' Deut. XV, 2.I.e., the creditor must inform the debtor of the release. ');">¹⁶</span>

Orchot Tzadikim