why the currency data(such as USD/JPY) is different from different source
A day is a long time and the rate is not the same all day. Some sources will report a close price that averages the bid and ask. Some sources will report a volume-weighted average. Some will report the last transaction price. Some will report a time-weighted average. Some will average the highest and lowest prices for the interval. Different marketplaces will also have slightly different prices because different traders are present at each marketplace. Usually, the documentation will explain what method they use and you can choose the source whose method makes the most sense for your application.
Are you talking about printing up more of the same kind of bill, or printing up a different kind of bill?  You'll have different answers based on which one you mean. If it's a different kind of bill: Governments don't like competition in this matter. In US history there are examples of the government shutting alternative currencies down.  A recent run at an alternative currency is the Liberty Dollar.  The similarity is not lost on BitCoin or even Chuck E. Cheese (last one is a satire, but I did worry for a second as I still have a bunch of those tokens!). If it's the same kind of bill: The currency is a tool of the government (in the US) and it does the sourcing for its production.  There isn't a whole lot of reason for others to get involved, really.  It's special paper, special plates, special presses, special everything, and doing it in one place ensures some consistency of product.  There aren't any compelling reason to open up another manufacturing channel to produce exactly the same product. There's no real economic benefit for banks to print their own money.  The larger ones play a key role in shaping how much is printed, but actually printing the bills is an offshoot of this. It depends upon in how many currencies your business is denominated. If your business is solely dependent upon this one payer, it's best to start up a new set of books in USD.  All accounts should be translated from CAD from a date preceding the USD activity.  The CAD books should be closed, and all should be done with the new USD books. If your business will continue to use both USD & CAD, it's best to have two sets of books, one for USD and one for CAD.   Multi-currency books are a nightmare and should be avoided at all costs. Also, with the way you describe your situation, it appears as if you're also blending your household and business books.  This should also be avoided for best practices. For manual conversion you can use many sites, starting from google (type 30 USD in yuan) to sites like xe.com mentioned here. For programmatic conversion, you could use Google Calculator API or many other currency exchange APIs that are available. Beware however that if you do it on the real site, the exchange rate is different from actual rates used by banks and payment processing companies - while they use market-based rates, they usually charge some premium on currency conversion, meaning that if you have something for 30 dollars, according to current rate it may bet 198 yuan, but if he uses a credit card for purchase, it may cost him, for example, 204 yuan. You should be very careful about making difference between snapshot market rates and actual rates used in specific transaction. "When you hold units of the DLR/DLR.U (TSX) ETF, you are indirectly holding U.S. dollars cash or cash equivalents. The ETF can be thought of as a container. The container gives you the convenience of holding USD in, say, CAD-denominated accounts that don't normally provide for USD cash balances. The ETF price ($12.33 and $12.12, in your example) simply reflects the CAD price of those USD, and the change is because the currencies moved with respect to each other. And so, necessarily, given how the ETF is made up, when the value of the U.S. dollar declines vs. the Canadian dollar, it follows that the value of your units of DLR declines as quoted in Canadian dollar terms. Currencies move all the time. Similarly, if you held the same amount of value in U.S. dollars, directly, instead of using the ETF, you would still experience a loss when quoted in Canadian dollar terms. In other words, whether or not your U.S. dollars are tied up either in DLR/DLR.U or else sitting in a U.S. dollar cash balance in your brokerage account, there's not much of a difference: You ""lose"" Canadian dollar equivalent when the value of USD declines with respect to CAD. Selling, more quickly, your DLR.U units in a USD-denominated account to yield U.S. dollars that you then directly hold does not insulate you from the same currency risk. What it does is reduce your exposure to other cost/risk factors inherent with ETFs: liquidity, spreads, and fees. However, I doubt that any of those played a significant part in the change of value from $12.33 to $12.12 that you described." Check the answers to this Stackoverflow question https://stackoverflow.com/questions/754593/source-of-historical-stock-data a number of potential sources are listed I think you answered yourself. The rate difference is because of the base rates differences, and it's not worth moving money around because the rate conversions (even if the currency rates don't fluctuate) will eat up the difference. "The solution I've come up with is to keep income in CAD, and Accounts Receivable in USD.  Every time I post an invoice it prompts for the exchange rate. I don't know if this is ""correct"" but it seems to be preserving all of the information about the transactions and it makes sense to me.  I'm a programmer, not an accountant though so I'd still appreciate an answer from someone more familiar with this topic." The interpretation is correct. The Reuters may give you the London 4PM rates if you query after the close for the day. The close rate is treated as the rate.  http://uk.reuters.com/business/currencies/quote?srcAmt=1&srcCurr=GBP&destAmt=&destCurr=USD The London 4PM rate may be obtained from Bank of England at the link below; http://www.bankofengland.co.uk/mfsd/iadb/index.asp?Travel=NIxSTxTIx&levels=1&XNotes=Y&XNotes2=Y&Nodes=X3790X3791X3873X33940&SectionRequired=I&HideNums=-1&ExtraInfo=false&A3836XBMX3790X3791.x=4&A3836XBMX3790X3791.y=3 Or any other Bank that provides such data