Rambam laws of Trumah ch. 3 law 21 and ch. 5 law 13

The first tithe can be obligated in Trumah in a case where it was given to the Levite from grain that was still obligated in Trumah. Butin general, the first tithe is not obligated in Trumah. It makes sense to say that that first tithe can be used to separate from it Trumah for other grain that is obligated in Trumah. [That is from the Sifri Zuta. Rabainu Shimshon brings this in Tractate Truma chapter 1, mishna 5.] But when it is not obligated in Trumah but still obligated in Trumat Masar, then it can only be used to separate from it Trumat Masar for other grain that is obligated only in Trumat Masar. I think this distinction is the opinion of the Rambam and it answers the contradiction between the Rambam in chapter 3 law 21 and chapter 5 law 13. and the langue of the Rambam in law 21 where he writes, “It can be used as Trumah until all of it is Trumah.” How could this be possible if the meaning of Trumah there means Truma Gedola? It is clear that the meaning is it is used for Trumat Masar for other grain until the last ten percent is left when that is also made Trumat Masar for its own obligation. This approach is different from Rabbainu Shimshon who holds that Masar Rishon which is still obligated in Trumat Masar (but not obligated in Truma) can be used to separate from it Trumah for other grain. [Later note. After writing the above paragraphs I noticed that Rav Shach writes part of what I wrote. That is, he takes the case that the masar rishon is obligated only in trumat masar and says that then only trumat masar can be separated from it for other grain and that is the meaning of chapter 1 law 21. I agree with this, but I add that a case in which the masar rishon is obligated in both truma gedola and trumat masar it can be used for both until the last ten percent is left. Then at that point, one needs to separate the remaining then percent as trumat masar for its own obligation.] ------The מעשר ראשון can be obligated in תרומה in a case where it was given to the לוי from grain that was still obligated in תרומה. But, there also can be the מעשר ראשון that is not obligated in ,תרומה . The רמב''ם brings such a case in laws of תרומה chapter 1.to me it makes sense to say that that first tithe can be used to separate from it תרומה for other grain that is obligated in תרומה. but when it is not obligated in תרומה but still obligated in תרומת מעשר, then it can only be used to separate from it תרומת מעשר for other grain that is obligated only in תרומת מעשר. I think this distinction is the opinion of the רמב''ם and it answers the contradiction between the רמב''ם in chapter ג' law כ''א and chapter ה' law י''ג. and the רמב''ם in law כ''א where he writes, “It can be used as תרומה until all of it is תרומה.” How could this be possible if the meaning of תרומה there means תרומה גדולה? It is clear that the meaning is it is used for ֱ for other grain until the last ten percent is left when that is also made תרומת מעשר for its own obligation. This approach is different from ר' שמשון who holds that מעשר rishon which is still obligated in תרומת מעשר but not obligated in תרומה can be used to separate from it תרומה for other grain. [Later note. After writing the above paragraphs I noticed that רב שך writes part of what I wrote. That is, he takes the case that the מעשר ראשון is obligated only in תרומת מעשר and says that then only תרומת מעשר can be separated from it for other grain, and that is the meaning of chapter א' law כ''א. I agree with this, but I add that a case in which the מעשר ראשון is obligated in both תרומה גדולה and תרומת מעשר it can be used for both until the last ten percent is left. Then at that point, one needs to separate the remaining then percent as תרומת מעשר for its own obligation.]