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 pain. It will say there is a payment against that customer, do you want to change the name, click 'Yes', then 'Record'. This will show the existing invoices as 'Not Paid', don't worry about this for now (you can apply a payment for each one as you go, but it takes twice as long, see 5 below).
4. Rename the original customer to something different, I suggest putting 'Old' at the start of their name. e.g. "Old Fred Smith Ltd"
5. Rename the new customer to that of the old customer e.g. from "2 Fred Smith Ltd" to "Fred Smith Ltd" - this should automatically update all the invoices you put in the new customer name, effectively putting them back in the name of the original customer
6. Open 'Customer List' and right-click on the 'new' customer, and select 'Apply Payment', it will come up with the list of invoices and the outstanding total, just click 'Record Only', and all the invoices will have a payment for the correct amount against them.
7. Delete the original customer, the one you have renamed to "Old....."
8. Back up your data!