Changing Sales Invoices after they have been printed is a really bad idea. I work with businesses who use specialised software for their industry to manage their business. But this software does not manage their accounts. So the Sales invoices get exported from this software and into their accounts software which is Sage Line 50. The problem is that neither the accounts software or the specialised software checks to see if the invoices were changed after the invoices were transferred to the accounts software.
A situation has now arisen where the customer has paid one amount and the accounts system is saying they owe more. What happened here is that someone found a mistake in the sales invoice after the integration process was run (integration process = Sales Invoices transferred from specialist software to sage line 50 accounts software). There is no 100% correct way of ensuring that people do not change invoices after the integration process has been run. People make mistakes. At least when the customer pays you, you will see a difference in value between what they think they owe you and what the sales invoice you have on file says they owe you.
The other thing people should get out of this is that they need to have a printed version of every invoice they send to their customers so that they can actually see what the client received. Because if someone changes the invoice you need to be able to explain why they did this and find out how it will affect the customer. Changing sales invoices after a customers has got an invoice is going to be hard to explain to a customer if they should have paid you more. It could cost you a customer!