Create a desktop program for a hardware supplier

I need a desktop program that will have a masterlist of pricelist for every company we supply to.

This masterlist will include a master price, a unique Discount Percentage ( unique for each company we supply for), a unique Invoice Number (per Sales order)

The program must also allow a manager to update the Unique Discount, the updated unique discount need to be in effect ASAP (as like "saving" a document) and must reflect which day it was last updated.

The program must be able to calculate the Item Master price x Unique Discount x item quanity
and add up the total items order per sales invoice.

This program must also allow editing of the Sales Invoice (even after printing) if additional orders were to be made by a customer.

The program must also be able to calculate all of the  invoice's total amount (for every company) to create a total Credit amount and Debit amount for every company we supply for.

In essense. this is exactly like a program that Food suppliers have for their restaurant clients