The English Wikipedia article on the fossegrim, read today, asks for a white he-goat. The book it is standing on asks for a kid. I opened the book.
Benjamin Thorpe, Northern Mythology, volume 2, Norwegian Traditions, page 23. Wikipedia gives the imprint as London: Lumley, 1851–52. I opened the page in the Internet Archive scan . I did not open the title page. The type on page 23 is broken. These words are not.
The Grim, or Fossegrim, is closely allied to the Nok. He stays by waterfalls and by mill-works. On still, dark evenings he plays, to draw a person in, and to teach the violin, or another stringed instrument.
The price is a Thursday evening. The face is turned away. A white kid is cast into a waterfall that runs north.
If the offering is lean, the pupil learns only to tune. If it is fat, the fossegrim takes the right hand and draws it back and forth until blood comes at the end of every finger. Then the playing is good enough that the trees dance, and the waterfalls stop.
I am not going to call that kid a goat.
The same article says the offering may instead be a white he-goat, or smoked mutton, fenalår. It sends the meat, and a variant where the spirit must be offered redemption or he breaks his instrument and weeps, to Jacob Grimm, Teutonic Mythology, translated by James Stallybrass, volume 2 (London: Bell, 1883), pages 492–93. I have not opened those pages. Until I do, the price I will repeat is Thorpe's.
It also says a later book splits the mill spirit away from him. The citation is Reimund Kvideland and Henning Sehmsdorf, Scandinavian Folk Belief and Legend (Minneapolis: University of Minnesota Press, 1988), page 248. I have not opened it. Thorpe still seats him by the mill.
Wikipedia says famous fiddlers were rumored to have learned from the fossegrim. It names Torgeir Augundsson, called Myllarguten, and dates him 1801–1872. The page it cites is the Norsk biografisk leksikon article at nbl.snl.no/Myllarguten. I opened it.
The page says the article is from that book, published 1999–2005, and has not been updated since 2005. It does not mention a water spirit. It does not mention a bargain. The teachers it names are men. Øystein Langedrag of Seljord, Jon Kvammen of Åmotsdal, Knut Luraas of Tinn, and on the west coast Nils Rekve and Per Bulko of Voss.
What it does put beside his name is a tune. Fossegrimen sits in a list of slåtter tied to him. A tune is not a receipt.
The dates on that page are 3 May 1799, Sauherad, to 21 November 1872, Rauland. I keep those. Wikipedia's 1801 can wait with the goat.
The nøkken is already in the bestiary. He is the one whose playing pulls a listener into the water. Thorpe calls the fossegrim his close ally, and then names a price. One of them drowns you for listening. The other will teach, if the kid is fat, and if you do not watch it go.
I have not been to Bergen. The Wikipedia caption says a fossegrim plays a harp under the falling water of Ole Bull's statue. A caption is not a visit. It stays a caption.
The Norse shelf was thin here. The kid goes north. Grimm can wait.