I don't know if it's considered a possibility by anyone who actually knows anything about English etymology - while a Google search for 'penny' and 'Penda' comes up with loads of sites, none of them seem to give any sources. Neither the Oxford Dictionary of English Etymology nor Duden's Herkunftswörterbuch suggest it, and it seems unlikely that it would have been borrowed into Old Saxon or Old High German from Old English (although it might have been borrowed in Old Norse). Its forms in the various early Germanic languages suggest a common West Germanic *panning, *paning or *panding, which look rather less like 'Penda' and suggest that it was probably a loanword. The ODEE suggests a link with pawn, which may be from Latin pannus, piece of cloth, while Duden suggests a a derivation from early forms of the word pan. 'Penda' seems pretty unlikely though.