Option 1:
Train your users better and explain to them why entering correct data in the system is important. Speak with their supervisors and make a plan with them how to improve the quality of transactional data. Make sub-dealer code mandatory. If dummy codes still appear in the reports after trainings and meetings, send the report with dummy sub-dealers to their managers. Usually this does the trick. 
@SR Kumar - in standard systems text fields are editable even after the invoice is released to accounting, probably in your case this is a custom development.
Option 2:
Updating all information for sub-dealers at a later time - I guess somebody outside of the company will provide the correct info. This means to drop the partner function concept for sub-dealers completely, because there should be a single source of truth.
Discuss with your developer an automated way to populate the data to a z-table.
What will be the best way to do that - well... this is why you get qualified developers - your colleague should be able to suggest the most appropriate solution.