Rabbeinu Nissim ben Re'uven in Maseches Pesochim (page 23 in the Rif ד''ה דילמא see here) says that, whilst one has to eat the Pesach offering "על השובע" (an eating of satisfaction), when it comes to matzoh and moror, "אי בעי הכי ועי בעי הכי", i.e. you can do what you like (על השובע or not על השובע). He then gives the source of this as being the Sifrei (although in the Oz Vehodor edition the source in brackets is Mechilta Parshas Bo Paragraph 6 see here). Let's assume that the source really is the aforementioned Mechilta. There, the midrash clearly states:
על מצות ומרורים יאכלוהו מכאן אמרו, הפסח נאכל אכילת שובע ואין מצה ומרור נאכלים אכילת שבע
"With matzoh and moror shall they eat it": From here they ruled: The Pesach is to be eaten in a state of satiety, but matzoh and moror are not to be eaten in a state of satiety.
This seems to be a direct contradiction to what this source was seemingly brought to prove, i.e. that matzoh and and moror may be eaten על השובע or not, depending on what the eater wants.
So the question is: If the source of Rabbeinu Nissim is the aforementioned Mechilta, then how can we resolve this apparent contradition between the Mechilta and Rabbeinu Nissim, and if the source of Rabbeinu Nissim is not the aforementioned Mechilta, then what is his source?
It is interesting to note that Rabbeinu Shmuel ben Me'ir (on page 99b of the Gemora ד''ה משום חיובא דמצה) agrees with the Midrash when he says:
משום חיובא דמצה - שלא תהא נאכלת על השובע וחיובא דמצה לילה הראשון חובה דכתיב בערב תאכלו מצות
Because of the obligation of matzoh- so that you do not eat the matzoh with satiety. And the obligation of matzoh on the first night is an obligation, as it says "on the evening (of the first night) you shall eat matzoh."
