The ברכה to יעקב begins ויתן לך האלהים מטל השמים ומשמני הארץ כ"ז:כ"ח. One might be tempted to translate the last phrase as "from the fats of the land," with the מ"ם as a prefix meaning "from." However, if that were the case, there should be a דגש in the שי"ן. But there is not. It therefore seems to be that the מ"ם is in fact part of the word - משמן. The same would apply in עשו's ברכה in פסוק ל"ט although it is harder to understand in context there.
The Minchas Shai brings down old manuscripts that in fact did have a dagesh in the shin, then dismisses these versions. He quotes the Eben Ezra and the Radak who both say that the "מ" of "מטל" applies to both words, implying that the "מ" of "משמני" is not one of שימוש. Then he quotes the Chizkuni who seems to imply that the "מ" is indeed a שימוש here.