Hello @mimiscottage,
You don't have to split any amount. Let me help share a few insights about how the parents and sub-accounts account set up and how your transaction is posted.
When you've assigned a sub-account under a parent account, the transactions in each sub-account will accumulate the total balance of the parent account.
Since you wanted to post the promotional credit into the sub-account register, consider changing the affected account into one of your sub-accounts so it'll post a credit into the corresponding register.
Additionally, I've also included this helpful article for the steps you need to take once you need to match the balance of your bank or credit card account with your monthly statement: Reconcile an account in QuickBooks Desktop.
You can always leave a comment below if you have any other questions. Stay safe!