@MoriDoweedhYaa3qob Believe me, I respect mori qafi7's opinion on all matters relating to the Rambam very highly, and I of course know that he changed his opinion regarding קידושי כסף. However, in this particular case I don't think you can refer to הלכות למשה מסיני as דברי סופרים, despite Qafih's version of the letter you are quoting. Sometimes, I'm afraid, we'll have to be מקבל את האמת ממי שאמרו and the alternate version of the letter makes much more sense.

If you can access them, I'd recommend looking at Havlin's book מסורת תורה שבעל פה pg. 90-93 and this article: daat.ac.il/daat/kitveyet/hamaayan/Hamaayan_83.pdf