When the IRS receives a copy of this 1099-B, they will see that you sold €50,000 worth of bitcoins on Cryptocurrency Exchange B. However, they will not be able to see that your true capital gains for this transaction were in fact less than €30,000. It will be up to you to prove that your base cost was in fact €20,000. If you can't prove it, you could be stuck with a zero cost basis and face a €50,000 capital gains tax bill.