חומש on פחות משוה פרוטה

Rashi and many of the Rishonim (see for example תוס' ד"ה אין) understand that everyone agrees that there is a חיוב חומש on a פחות משוה פרוטה the question is can you be פודה the מעשר שני if the חומש is less then a שוה פרוטה

אחרונים ask according to the מ"ד who holds that חומש is not מעכב why wouldn't you be able to be פודה if the חומש is less then a פרוטה after all the פדיון is חל even if he doesn't give the חומש at all? In other words, we should say that ר' אסי and ר"ל hold that חומש is מעכב. One answer may be this is like כל הראוי לבילה אין בילה מעכבתו. The חומש is not מעכב if you can give it. However, if it is less then a שוה פרוטה and therefore you can't give it then everyone would agree that it is מעכב.

The Rambam has a different understanding of this Gemara. The Rambam (מעשר שני ה:ד) understands that the מחלוקת is whether there is a חיוב חומש of less then a שוה פרוטה and ר' אסי and ר"ל hold that if the מעשר שני is worth less then 4 פרוטות you don't need to add a חומש. In fact, the מפרשים point out that the Rambam is a בפירוש ירושלמי however the לשוןof the ברייתא is difficult for the Rambam. The ברייתא explicitly mentions that הוא וחומשו even though according to the Rambam he doesn't need to pay the חומש.