Are you here because you sent $50 in bitcoins but have received only $40?
The simple explanation is that the value of Bitcoin is very volatile at the moment, and the price is constantly changing to market conditions. If you have BTC 1 in your Bitcoin Wallet, this means that your balance is equivalent to 1 Bitcoin.
The equivalent value in fiat currency (GBP, EUR, USD, etc) is indicative of the changing value. This fiat value is always fluctuating: for example, Bitcoin was valued at $2170 on July 17, then $7380 Nov 3, and then $6008 on Nov 16.
You still own BTC 1 nonetheless. When you send bitcoin to another bitcoin address, you are sending the Bitcoin value, not the fiat value. E.g. If you are sending John Doe BTC 0.3, he will receive BTC 0.3 (minus blockchain fee).
When you make an exchange between Bitcoin to Fiat (Bitcoin to USD, USD to Bitcoin, Bitcoin to EUR, GBP to Bitcoin, etc), we will inform you about the exchange rate (based on W-PAY), to allow you to review and confirm your action before proceeding.
Learn more about the Common Misconceptions of Bitcoin Pricing on our blog.