That's called prostitution. One word. There. Was a 4-line paragraph needed to explain what you're doing?

Nonetheless, it's a good idea.

However, that's not your slave. Perhaps part-time submissive... The status of slavery implies that the slave is totally devoted and available at the desire of the master and the amount of time for which she/he is ready for you is not conditionned by some new action from the master (such as giving her money, gifts, etc). At least this is how I see it.

I'm not saying that buying her with gifts has no place in slavery, I'm just saying that she's not a slave if this buys you play time :-)