Reference for Nedarim 66:7

וחד אמר אפילו נכסי בעל אבידה אסורין על מחזיר מהדר ליה ומשום פרוטה דרב יוסף לא שכיח

AND RETURN A LOST ARTICLE TO HIM. R. Ammi and R. Assi [differ thereon] — one said: This is only when the property of the finder<span class="x" onmousemove="('comment',' Lit., 'restorer'. ');">⁷</span> is forbidden to the loser, so that in returning it to him, he returns what is his own.<span class="x" onmousemove="('comment',' So that the loser is not benefiting. ');">⁸</span> But if the property of the loser is forbidden to the finder, he may not return it, because he benefits him by R. Joseph's <i>perutah</i>.<span class="x" onmousemove="('comment',' Since when a person is engaged in the performance of one precept, he is exempt from another, the finder, when fulfilling this precept, may decline to give a perutah of charity to a poor man. This is referred to as R. Joseph's perutah, because he based a certain ruling upon this fact. B.K. 56b. ');">⁹</span> But the other maintained: Even if the finder may not benefit from the loser's property, he may return it, and as for R. Joseph's <i>perutah</i>, this is rare.<span class="x" onmousemove="('comment',' One rarely avails himself of that privilege, hence the finder gains nothing. ');">¹⁰</span>

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