Talmud: Bava Kama 104

Thank you, Rabbi! A question: In 104b, the gemara resolves that (according to Shmuel), one way in which a creditor (or in which the owner of an entrusted object) can reclaim their money is by gifting the debt (or the pikadon) to a third party, together with a small parcel of land. When that third party goes to collect the money, on what basis does the debtor (or shomer) know that they really own it? If they bear a letter to that effect, signed by witnesses, how is this any different to a דיוקני? And if they're coming with the approval of a beys din, why not just cut out the whole acquisition process and make them a shaliach beys din from the beginning? Reply