No, that wouldn't work, as the program thinks it is correct.
There is a way to do this, but it is an almighty faff BECAUSE EXPRESS INVOICE WON'T LET ME ENTER A NEGATIVE NUMBER, which would easily resolve it.
The way to correct this is:
1. Back up your data!
2. Create a new customer, make it very slightly different to the one with the wrong balance, e.g. use the same name with an A or 2 in front of it, so if the customer is "Fred Smith Ltd", use "2 Fred Smith Ltd"
3. Change ALL invoices to the name of the new customer. This can take a while, and is therefore a bit of a