Choose the business rounding boundary
Choose a rounding rule at the business boundary where a fractional minor unit must be resolved, and preserve the original amount, currency, and relevant rate or quote when later audit depends on them. For invoices and payments, snapshot the agreed amount so a changed catalog price cannot rewrite an existing obligation. Display formatting does not change stored precision.
The language's exact numeric and money helpers are listed in types and builtins. External settlement is a separate outcome from a successful business transaction; see payments and deliveries and recovery.