In this quest, you ultimately find two Black Soul Gems in Falcar's dresser drawer. I've tried turning only one over to Deetsan, but she still refers to it in the plural ("Oh, dear - Black Soul Gems!" and "I'll just take these."). And, once she takes it, I return to the dresser to find the other one gone from the drawer. According to the Unofficial Elder Scrolls Pages website, these are props, anyway - actually Grand Soul Gems, which can't be used to trap NPC souls.
So, I use the Console to correct the programming deficiencies that have GSGs masquerading as BSGs, and that make it impossible to hold one BSG back. After giving one to Deetsan, I then hit the tilde key, and type "player.additem 192 1" (without the quotes), and hit RETURN. And, like magic (a little pun, there), I have the other, empty, Black Soul Gem in my inventory. Just expanding my action choices in the story.
BTW, speaking of Console commands: assuming that an item which you enchant would have one, is there a way to find out that item's ID number, such as is used in Console commands? For instance, once I enchant a shirt with Chameleon 20%, or a bow with Drain Health 100 for 1 second, can I find out what those newly-enchanted items' ID numbers are - if they have them?