Currency conversion that goes off-budget is just fine:
2022-01-02 * "Blah" Assets:Checking -90.00 EUR Expenses:Groceries 110.00 USD @@ 90.00 EUR
It gets complicated for an on-budget currency conversion:
2022-01-02 * "Other blah" Assets:Cash -55.00 USD @@ 45.00 EUR Assets:Checking 45.00 EUR
For this to be a valid transaction in a budget, the foreign currency must be already budgeted for (55.00 USD) and the money gained from currency conversion (45.00 EUR) would go into inflow.
In Beancount, the
Equity:Conversions:Current account is used for this.