"He always buys his food with bills."
Translation:Il achète toujours sa nourriture avec des billets.
Can we have "notes", or even "cash" for the Brits here? In England a "bill" is not really something you can use to buy food, unless you mean by extension from the fact that bills you issue to other people constitute your wages when paid. Literally speaking, a bill in England is the total that is being demanded for payment, not used to pay - an invoice/appel de fonds.
I don't think it matters what word we use in our own language although it would be easier if it did. We are learning to speak French and if that's the word used in French that's the word. It works the other way too, I had a roommate who was French. He would talk about going to "broom his room"
In America we use the word "bills" to mean "notes", as the Brits use it. However, we never use it all by itself, as it appears in this sentence. We say "one-dollar bills" or "five-dollar bills", but never "bills". "Cash" would be the best alternative to "bills" for American English in this case.
The bills are always LES. If in English you mean "some" or you wouldn't put any word before bills, as in this example, you use a variation of DE, the French partitive article. There are many rules around de, which you can read about here: http://french.about.com/od/grammar/a/articles_4.htm
As you may know, we have had Euros across Europe for 13 years, which kind of broke our previous familiarity with our respective currencies. The first Euro note/bill is 5 euros. Below, we have coins and I don't know of any nicknames attached to any of these.
In the old days, the slang for "French francs" was "balles" (why? I don't know): "ça coûte 100 balles" meant 100 francs. Nowadays, I tend to use "balles" to mean Euros, but I may not be representative of the French population in that respect.