Shibuda d'Rav Nassan - creating a chov with no assignee

Our parsha contains the source for the halacha known as "shibuda d'rav Nassan". The Torah says (Bamidbar 5:7) that stolen property should be returned "l'asher asham lo", to whom the principal is owed. Why does the Torah use this circumlocution instead of saying return the property to the person from whom it was stolen? From here R' Nassan (Kiddushin 15, Pesachim 32) darshens that we are speaking of a case where the debt has been assigned and is owed to someone other than the person the money was taken from, e.g. if Reuvain owes money to Shimon and Shimon owes money to Levi, Reuvain would pay Levi directly.
The Torah Temima writes that from shibuda d'rav Nassan we learn that the creditor or assignee of a debt need not be specified at the time a debt is incurred. Using the example above, when Reuvain took his loan from Shimon, he had no way of knowing that his debt would be assigned to Levi, yet the debt is still binding and collectable by Levi. It seems to me that there are two problems with this proof, one a ba'ale batish chiluk, one more lomdish: 1) In the example we used (which is the classic case), at the time the debt (or shibud) is created, it is payable to a specific party. True, shibuda d'rav Nassan tells us a creditor can tranfer a debt to his/her assignees, but isn't that different than the T.T.'s case of creating a debt with no creditor named? 2) The classic question raised in discussing shibda d'rav nassan is whether (using our example) Reuvain is considered the debtor of Levi, or Reuvain remains the debtor of Shimon, but that debt is satisfied by payment to Levi - i.e. shibuda d'rav Nassan is the equivalent of payment instructions, but not an actual assignment of the loan to some third party. It seems to me that the T.T. has no proof at all if one assumes that Reuvain's debt is never actually transferred to some party he was never aware but always remains bound to the original creditor known at the time of the debt's creation.