Source and decoding
לא קשיא: הא — דקביל עליה אחריות, הא — דלא קביל עליה אחריות
It is not difficult: this is where he accepted responsibility, and that is where he did not accept responsibility.
The baraita says one may see others' chametz yet may not accept deposits from a non-Jew, and the Gemara resolves the contradiction by distinguishing accepting responsibility from not accepting it. Where he accepted it, as Rava told the people of Mechoza, the chametz stands in his possession and he must pay if it is stolen or lost. The Gemara goes on to ask whether 'something that causes monetary liability' counts like money, so not every case is the same.