Kesubos 101b - Admitting to Owing Money

Rashi explains that the machlokes between R' Yochanan and Reish Lakish is speaking about a situation where reuven is admitting to a preexisting loan that he owes shimon, and the machlokes is when he admits in front of eidim, whether the eidim can now testify that reuven actually owes the money. The gemara then says that if reuven told the eidim "a'tem eidai" then all agree that reuven is responsible to pay, and if he did not then all agree he is not responsible to pay. The case of argument is where reuven writes shimon a contract that says he owes him $100. Rashi seems to be bothered how this case would be different than a case where one signs a contract admitting to a preexisting debt, where the contract would definitely be binding. Rashi explains that we are speaking about a case where there is no signature, rather reuven writes "i owe you $100" and hands it to shimon in the presence of witnesses. Rashi seems to imply that even at this stage of the gemara we are speaking where reuven is admitting to a preexisting loan, but if he is trying to create an obligation through this contract that did not exist before, it would not be binding. The Ran agrees with rashi in the first stage of the gemara but then switches to explain this stage of the gemara to be referring to a case where reuven is using this contract to "create" a new obligation, not an admission to a preexisting debt. The Rambam (as explained by the Ran) learns the entire gemara from beginning to end to be referring to a case where reuven is trying to create a new obligation by either telling shimon that he owes him money in the presence of witnesses, or writes a contract who's purpose is to create this obligation. The Rambam's opinion seems clear that admitting can to much more than serve as evidence to a preexisting debt, it can even create a new obligation that did not previously exist.