מחוי ליה ר' אבהו ובשבועה (Bava Metzia 7a)

Is the שבועה on the whole thing (including what he holds) or is the שבועה only on the rest (what he is not holding)?

Tosafos (ד"ה מחוי) states that the שבועה is on all of it. The Rambam holds that the שבועה is only on the rest. The Rambam would seem to be supported by the Gemara. Since what you are holding is כמאן דפסיק דמי then why should you need to take a שבועה on it it is yours?