In the Rambam's Introduction to the Mishnah, he mentions a Halacha L'Moshe Mesinai regarding mixing different sorts of wine. The quoted text is ביין התירו לערב קשה ברך הלכה למשה מסיני. Does anybody know what this law is?
The source is Mishnah Bava Metzi’a 4:11.
בֶּאֱמֶת, בְּיַיִן הִתִּירוּ לְעָרֵב קָשֶׁה בְרַךְ, מִפְּנֵי שֶׁהוּא מַשְׁבִּיחוֹ.
In truth they permitted one to mix hard wine into soft, as it improves it.
As explained there by Bartenura, if one agrees to sell soft wine, he may mix some hard into it, as that increases the value of the product he is selling.