The balance out is equal to purchases minus paid. Similarly with balance in, it’s sales minus received (e.g. the total outstanding).
It’s likely that this euro invoice is the one that’s showing up here. As I mentioned above, the figure used in the reports is the GBP equivalent based on the invoice.
For example, I’ve raised a test invoice for €100.00, equal to £88.59. When I’ve paid this, I’ve marked it as the €100.00 being worth £85.00 (£3.59 difference), which shows on the project report:
Bear in mind however, the project report doesn’t take into account VAT. So if that €100 included VAT, the figure would be different again.