Each amount field on the invoice window points to an internal number in the GL Account file.
To see this link go to Ledger > Account Categories.
Find Account 4110, or your sales discount account.
Notice the lower left corner, the “Internal Number”.
This number is used to find this account when posting an invoice.
You can change the (User) Account number to whatever you want, and discounts will still post to the account with the internal number 4110.
The same steup is used for Shipping, Taxes, and, if invoice line item posting is not turned on, Sales.