Base currency is disabled for a resource

Symptoms:
Base Currency field in User Setup page is disabled for a resource and cannot be modified.

Reason:
Base currency is disabled if the resource has outstanding expense advances (i.e. the advance balance is greater than 0) or has unapproved expenses.

Resolution:
1- Create an expense equal to the outstanding expense advance amount,
2- Create an expense report, including both this expense and the full advance amount (by selecting the "Apply total advance" option).

After all the expense reports are approved, including the new expense report created for the outstanding advance amount, Base Currency field will be enabled.