We are an established Hospital in Mexico, and we would like to accept
credit card payments
through a payment processor/merchant account.

We would be charging Medical services and Pharmacy (Retail: Face to Face).

My question is:
If I open a Merchant account in the US, do I have to pay TAX in the US?

Unless you're doing business in the U.S., you don't have to pay taxes
in the U.S. Consider this analogy: it's the same as if you were to pay
AT&T for your phone service, yet operated solely within Mexico. Yeah,
they are helping you conduct your business, but the IRS isn't entitled
to cut of all foreign business that flows through U.S. corporate
assets. Heck, we'd have a lot more tax revenue if that were the case!

I hope this helps.

Thank you for your comment.

However, I would like a point to a law or something so I can be ensured of that.

Thank you

Well, the way the American tax system works is that you don't have to
pay taxes on anything unless the IRS says you do, subject to
exceptions as decided by case law, generally. The one thing that
*might* trigger taxes would be unearned income on interest generated
from assets left in a US merchant account. You could fix that by not
having an interest-bearing account though. Otherwise, you are simply
not generating revenue in the US. That merchant account is an expense 
for you ... they are the ones making money and you can rest assured
they are paying taxes on it.

I would love to be able to point you to a decision that says that
foreign companies with no US assets or US activities don't have to pay
US taxes, but it's just common sense I think.

Good luck. I wish I could be more help.

Cool, I'm creaing a lot of doubts.

However, as far as I know, every bank in the US asks you for a TAX ID. Am I wrong?

Yep, every bank requires you to submit a tax id. Having a tax id
doesn't mean you are obligated to pay tax, however.

On a separate note, take a look at this passage from the IRS web site:

"Foreign persons are subject to U.S. tax at a 30% rate on income they
receive from U.S. sources that consists of interest (including certain
original issue discount (OID)), dividends, rents, premiums, annuities,
compensation for, or in expectation of, services performed, or other
fixed or determinable annual or periodical gains, profits, or income.
This tax is imposed on the gross amount paid and is generally
collected by withholding on that amount. A payment is considered to
have been made whether it is made directly to the beneficial owner or
to another person for the benefit of the beneficial owner."
-- http://www.irs.gov/instructions/iw8exp/ch01.html

You may want to retain a tax attorney or an accountant to clear this
up, but other than interest income, I don't see how you could possibly
be generating revenue in the US by conducting face-to-face retail
transactions in Mexico. None of those revenue types seem like they
would apply. However, it sounds like you need to have a formal
decision on the matter and not a comment by some guy on the internet.
Again, good luck.

Hi seewright,

I would like to arrange some type of agreement to you to obtain more
of your help. That help will consist on research, questions/answers.

I dont know how google answers work and if it is allowed that I
contact you outside the site.

Please let me know how can we get in contact.

thanks