If the Ganav is not found, the Shomer Chinam swears in front of Beis-Din and is Patur. What does he swear?
Rashi, Rashbam and Targum Yonasan: He swears that he did not take the article for his own personal use. 1
Ramban: He swears to substantiate his claim that it was stolen. 2 Gur Aryeh - Rashi on Pasuk 10 says so. Since the Torah suspects him of Shelichus Yad and makes him swear, he must swear also about how it was lost.
Because if he did, it is akin to theft and he would be Chayav even for subsequent Onsin (Ramban and Seforno, citing Bava Kama, 63b). Refer also to 22:10:1:1.
"Im Lo Shalach Yado ... " is the condition that enables him to swear and be Patur, not a description of the Shevu'ah. The Ramban agrees that he swears also that he was not Shole'ach Yad. See the final comment in Rav Chavel's footnotes.
What is Shelichus Yad?