Here is the technical guidance from the accounting standard FRS 23 (IAS 21) 'The Effects of Changes in Foreign Exchange Rates' which states:
Exchange differences arising on the settlement of monetary items or on translating monetary items at rates
different from those at which they were translated on initial recognition during the period or in previous
financial statements shall be recognised in profit or loss in the period in which they arise.
You agree to sell a product for $100 to a customer at a certain date. You would record the sale of this product on that date at $100, converted at the current FX rate (lets say £1:$1 for ease) in your profit loss account as £100. The customer then pays you several $100 days later, at which point the FX rate has fallen to £0.5:$1 and you only receive £50. You would then have a realised loss of £50 due to exchange differences, and this is charged to your profit and loss account as a cost. Due to double entry bookkeeping the profit/loss on the FX difference is needed to balance the journals of the transaction.
I think there is a little confusion as to what constitutes a (realised) profit/loss on exchange difference. In the example in your question, you are not making any loss when you convert the bitcoins to dollars, as there is no difference in the exchange rate between the point you convert them. Therefore you have not made either a profit or a loss.
In terms of how this effects your tax position; you only pay tax on your profit and loss account. The example I give above is an instance where an exchange difference is recorded to the P&L. In your example, the value of your cash held is reflected in your balance sheet, as an asset, whatever its value is at the balance sheet date. Unfortunately, the value of the asset can rise/fall, but the only time where you will record a profit/loss on this (and therefore have an impact on tax) is if you sell the asset.