A problem in Rambam

To the Grand Master,
The Rambam wrote (1:1, "Chametz and Matzah 59"): And the Sages forbade eating chametz from the beginning of the sixth hour, etc., therefore, leaven is placed in it, and so on. Leaven, which is holy, is not eaten or burned until the sixth hour arrives, and everything is burned, etc. And see the MMM, which is the source of the Rambam's text from the book of Pesachim (11:2) in the Matani. And Maran the Gerizh explains this, that the fifth hour is only a prohibition of the Gevara, of eating leaven from the Das, and one does not burn a Terumoth because of the observance of Terumoth, and only at the sixth hour, which is also a prohibition of the Haftza on leaven from the Das, and thus the observance of Terumoth, and one must burn the leaven. And here is the interpretation of the Rambam (ibid., 53:4): It is customary for a 14-day fast to be on Shabbat, since it was before Shabbat, and he had many loaves of terumah, and it is necessary to burn them on the Sabbath, therefore [its source is from Mittani in the Book of Pesachim (page 94), Damboer Minya, who says that Mishmaret Terumuti is in the Book of Pesachim, even when it is forbidden by the Lord to burn leaven, which is already burned on the 13th day of Nisan], and indeed in the Book of Pesachim (page 13) and in the Book of Torah (ibid.) it is implied that this is a disagreement between the two Mishnayots, and thus how does the Rambam rule that the two Mishnayots are an apparent contradiction.
And it seems that it is possible to explain simply that the Rambam held that when the Sages established the burning of leaven, they were aware that it could happen on Shabbat, and their basic rule was that on Friday the prohibition would apply in a haphazard manner so that they would not come to a mistake and one should not judge what is possible from what is impossible.
We will be happy for the enlightenment of Maran Rav Shlita,
With loyal greetings and intense love,
His student,

The Fire of David and Shabbat for David