bit late to the party here, but why not something where your character vows to only use the mace to thwart things that would actually help Bal thereby making the weapon utterly useless to him because yes hes getting energy from it but his plans are being ruined so hes more screwed by you having it than not. I dunno it just seems like a good way of screwing over daedra. alternatively you could also vow to just lock it away after you get it and never use it