A lot of retail establishments aren't taking $50 or $100 bills
 anymore.

 I can understand that breaking just a couple of those in an evening
 can deplete their small bills, but something bothers me - 
 I thought all US paper currency declares that it is 
 legal tender for "all debts, public and private."

 How can they legally refuse any tendered cash???

Dear pcventures-ga,

Unless there is a State law which obliges them to accept, then yes,
they can refuse if it is ?for goods and services not yet provided?.

The Department of The Treasury deals with this on their FAQs page on currency.

The relevant law is the Coinage Act of 1965, specifically Section 102.
and section 392 of Title 31 of the United States Code. "The law says
that: "All coins and currencies of the United States, regardless of
when coined or issued, shall be legal-tender for all debts, public and
private, public charges, taxes, duties and dues."

However, it continues that "There is, however, no Federal statute
mandating that a private business, a person or an organization must
accept currency or coins as for payment for goods and/or services.
Private businesses are free to develop their own policies on whether
or not to accept cash unless there is a State law which says
otherwise. For example, a bus line may prohibit payment of fares in
pennies or dollar bills. In addition, movie theaters, convenience
stores and gas stations may refuse to accept large denomination
currency (usually notes above $20) as a matter of policy."

Source
http://www.treas.gov/education/faq/currency/legal-tender.shtml

"However, no federal law mandates that a person or an organization
must accept currency or coins as payment for goods or services not yet
provided. For example, a bus line may prohibit payment of fares in
pennies or dollar bills."

Source
http://www.federalreserve.gov/generalinfo/faq/faqcur.htm


I hope this answers your question. If it does not, or the answer is
unclear, then please ask for clarification of this research before
rating the answer. I shall respond to the clarification request as
soon as I receive it.
Thank you
answerfinder

Search stragey
Information from the searches "cannot refuse"  "legal tender" led me
to search  "legal tender act" refuse
://www.google.co.uk/search?hl=en&q=%22legal+tender+act%22+refuse&btnG=Search&meta=

Not the answer I wanted (I was hoping I could compel merchants with
the force of law to accept any bill in my wallet) but it was the sort
of detailed explanation I was looking for.

Dear pcventures-ga,
Many thanks for the tip. I'll accept cash at any time!
answerfinder-ga

This causes me to wonder... what about goods or services already rendered?

Like pumping gas, before paying for it. Does this not create a debt?
Or paying at a resteraunt after eating.

I guess if there is a sign or statement, form of payment would be
included in the quid pro quo transaction.

<shrug>

A store here in Denmark I worked in didn't accept $100 bills, though
we accepted dollars in general. I was told that was because the $100
bill is the most counterfeited bill in the world.

This then begs the question: What states, if any, require the acceptance of cash?