I'm going to get my girlfriend an expensive present for her birthday instead of a card
Why do you want to get her a present?
She just needs to feel richer
If this is a gift that she really wants, there's no problem. Just make sure it is something that she wants if you are going to spend a ton of money on it.
She's my slave. That's not apart of this world and I can do whatever I want to her.
That escalated quite a bit. You shouldn't own slaves and use them like this. Free her instead of giving her a gift.