Hmmm...this is a tough one. It would be best if both of you wanted the dog but since that isn't the case, I don't feel it's right for your girlfriend to be denied a Yorkie if she really wants one and is committed to taking care of it. Yorkies crave human attention. Due to their small size, they are portable.
So if you and she need to run errands on Saturdays, your girlfriend can bring her along in a carrier -- that way, you'd have the best of both worlds.
I would not recommend just saying NO to the dog. I think if she really wants a Yorkie as badly as you say she does, she (and you) will find a way to make it work. The total unconditional love these little ones give you will outshine any adjustments you may have to make.
All the best of you to you both.