I don't know if this is the same everywhere, but when we bought our house,
one of the forms we had to fill out was a sworn affidavit that the money we
were using for the down payment was our own, and was "not borrowed in any
way."

Even if your house doesn't come with such a form, I would think that what
you're proposing to do is to borrow money at high interest in order to
reduce the amount of money you need to borrow at lower interest. Sounds
like a mistake to me.

You may not have seen my original post. My down payment is my own and
not borrowed, the whole 20%. The $10K of *extra* down payment would be
borrowed, and I'm pretty sure my banker would not have a problem with
that. I would get approval of course.

Also I said the credit card rate is *lower* than the mortgage, not
higher. 3.99% vs 6.375%.

After much consideration we are probably not going to do it. There are
other things we'll want to do to the house soon (make the garage larger
for one - I don't know how people get by with a regular two car garage)
and we don't want that extra $10k of debt preventing us from doing it.