I think that entry is a mistake in Lewis and Short that has since been fixed. Glare doesn’t include it in his OLD, and I don’t see it at the cited passage (the edition on PHI has pecunia there instead). The writer of your book was probably thinking of pecū, pecūs n., though if not in the ablative, it’s typically in the plural (Glare: “sg. in quots. only in abl.”). I guess it’s possible, if unattested, but pecus (pecoris) or pecua is the better choice.