We discuss for Kinyan Chalipin.
The question is about the Hava Amina that entails an additional teaching to exclude the possibility of Chalipin with coin.
The answer can be found in Kesef Mishne
Introduction: The rationale is that when the Gemara says mana, the definition is a not-fruit. So coin may be a mana in this sense. And further the Gemara found two different versions of the statements of Rav Papa. the first is that coin is not equivalent to utensils, and the second says that coin is equivalent to utensils in matter of Chalipin (Not in Matter of hilchot Shabbat or hilchot Tum'at Kelim).
See bellow a snippet of Kesef Mishne Sefer Kinyan, Hilchot Mechira, chapter 5, Halacha 6, annotated: The whole text is here
ואמרו בגמרא במנא לאפוקי ממ"ד קונין בפירות ביה למעוטי מטבע שכיון שאמרו במנא נתמעטו בין מטבע בין שאר דברים דלא הוו כלי וביה למה לי למעוטי מטבע?
י"ל דכל מילי מיקרו כלי לבד מפירות לפי שאין דומים כלל לנעל שאין משתמשים בהם בשום מלאכה ונרקבין ונפסדין במעט זמן ואין מתקיימין כנעל ומטבע כעין נעל הוא דראוי להשתמש בו לתלותו בצואר הבנות ולשקול בו משקלותיו ואינו נרקב [...]
And it is said in the gemara, "A utensil" to exclude the opinion
that holds that one can effect a chalipin with fruit, and
"therewith" to exclude coins. [One might ask] since it is said "with
a utensil," that should exclude both coins as well as anything else
that is not a vessel, and why do we need "therewith" to exclude coin?
One could answer that anything can be called a vessel besides for
fruit, because they are not at all similar to a shoe [see explanation
below], because they cannot be used for any labor. Additionally, they
rot and are lost after a short time and do not last like a shoe. And
a coin is like a shoe because one can use it for [some form of] work,
e.g. to hang it from the necklace of young girls, or to weigh things
with it, and it doesn't rot [...]
The reason we care so much about comparing the object of chalipin to a shoe is because the inverse opinion of the one that was excluded from "mana" (Rav Nachman) holds that because the source of chalipin is "ושלף איש נעלו" (Rus 4:7), chalipin only works with something that is "similar to a shoe," in other words, a "kli."