You can use the tags {amount} and {unit} in the Procedure Rule's Invoice Message field to correctly display the amount and unit of a procedure.
To do this, add the tags {amount} and {unit} to the Invoice Message field while creating or editing a Procedure Rule.

More information on creating or editing Procedure Rules can be found here:
The system will then use the amount and units chosen for the rule to be displayed where the tags are entered:

Please note that both {amount} and {unit} are optional and can be entered multiple times in Invoice Message field.