If you are referring to Daf 48, then presumably you are speaking about the Gemara of "Eravon," which is not discussing a loan, but rather a sale. The buyer gave a security deposit with the following stipulation: "If I renege I will forfeit my pledge; if you renege you will return double my pledge. Rebbi Yosi -- who holds that an Asmachta is binding -- holds that this condition is binding, Rebbi Yehudah -- who holds in general that an Asmachta is not binding -- holds that the condition is void.

The Halachah is accepted universally that an Asmachta is not binding. However, in this particular case there is a difference of opinion between the Rishonim whether both sides of the condition are considered an Asmachta, or merely the seller's stipulation that he will double the pledge is an Asmachta, whereas the buyer's stipulation to forfeit his pledge is binding and is not an Asmachta at all, the reason being that since the seller has possession of the object already, Asmachta does not apply.

The Halachah in this case, as you wrote, is a Machlokes between the Shulchan Aruch (which is the accepted Halachah for Sefardim) who holds the latter opinion, and the Rema (which is the accepted Halachah for Ashkenazim) who is of the former opinion. However, when it comes to monetary matters such as in this case, whoever has posssession of the item or money in doubt has the right to say that he accepts the position of the Rishonim who support his claim of, and we are not empowered to make him relinquish his possession.

D. Zupnik