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.