A noun appearing as N. salff “broth” in The Etymologies of the 1930s derived from ON. salpha “liquid food, soup, broth” under the root ᴹ√SALAP “lick up” (Ety/SÁLAP; EtyAC/SÁLAP). In The Etymologies as published in The Lost Road the form was salf (LR/385), but Carl Hostetter and Patrick Wynne corrected this to salff in their Addenda and Corrigenda to the Etymologies (VT46/12).
Neo-Sindarin: For purposes of Neo-Sindarin I would update the spelling of this word to ᴺS. salph “broth”, as suggested in HSD (HSD).
The form salf in the Etymologies is a misreading, see VT/45:12