Come on - that's not an obstacle. First, it's YOUR duty to report the income and the appropriate local currency value of that income, so picking a rate is your problem, not theirs.

And then, if they don't like your choice rate, then they pick a rate themselves, tax you on that, and fine you for misrepresenting your income.

Afterwards you (and everyone else) begs them to publish some guidelines on what rate should be used, they do so, and everyone is [mostly] happy.

What exchange rate do they use for forex transactions?