Bava Metzia around page 82, Pesachim page 31b

I have wondered why the Rif and Rambam ignore the differences mentioned in the Gemara about a pledge. For example, in Bava Metzia around page 82 the Gemara brings the Mishna that says a lender with the pledge is a paid guard. It asks if that is perhaps not like R Eliezer who says he is an unpaid guard. The Gemara suggests perhaps they are disagreeing about the law of R Isaac who says one who lends to another with a pledge owns the pledge. Answer. R Isaac refers to when he took the pledge at a different time than the time when he made the loan. [you can understand this like Tosphot who says R Isaac means he is a paid guard, and that refers to when he took the pledge at a different time than the time of the loan. You can also understand this like Rashi that R Isaac means he is a borrower if he took the pledge at a different time than the time of the loan.] The Rif however understands R Isaac to mean the lender is a paid guard and it makes not difference when he received the pledge. I think the Rif is based on the Gemara in Pesachim page 31b which bring a teaching that says if a Israeli loaned money to a gentile and received leavened bread as a pledge and it remained in his domain during Passover, he does not transgress any prohibition. However, R Meir said he does transgress. Thus, the Rif simply decided the law like the first Tana teacher (the first opinion in the braita). (That does seem not to be like R Isaac. To R Isaac the Israeli ought to be considered to transgress.) It is however unclear to me why the Rif did not say the argument between the first tana and R Meir when he took the leavened bread at the time of the loan; and R Isaac refers to when he took te bred at a different time.-----------------------------------I have wondered why the רי''ף and רמב''ם ignore the differences mentioned in the גמרא about a משכון. For example, in בבא מציעא around page פ''ב the גמרא brings the משנה that says a lender with the משכון is a paid guard (שומר שכר) . It asks if that is perhaps not like ר' אליעזר who says he is an unpaid guard (שומר חינם). The גמרא suggests perhaps they are disagreeing about the law of ר’ יצחק who says one who lends to another with a משכון owns the משכון. Answer. ר’ יצחק refers to when he took the משכון at a different time than the time when he made the loan. [You can understand this like תוספות who says ר’ יצחק means he is a paid guard, and that refers to when he took the משכון at a different time than the time of the loan. You can also understand this like רש''יthat ר’ יצחק means he is a borrower if he took the משכון at a different time than the time of the loan.] The רי''ף however understands ר’ יצחק to mean the lender is a paid guard and it makes not difference when he received the משכון. I think the רי''ף is based on the גמרא in פסחים page 31 which bring a teaching that says if a יהודי loaned money to a נכרי and received חמץ as a משכון and it remained in his domain during פסח, he does not transgress any prohibition. However, ר' מאיר said he does transgress. Thus, the רי''ף simply decided the law like the תנא קמא teacher (the first opinion in the ברייתא). (That does seem not to be like ר’ יצחק. To ר’ יצחק the יהודי ought to be considered to transgress.) It is however unclear to me why the רי''ף did not say the argument between the תנא קמא and ר' מאיר when he took the חמץ at the time of the loan; and ר’ יצחק refers to when he took החמץ at a different time.