Can I send an invoice that shows we have applied an outstanding credit memo and he only owes the difference?

I think it would reflect correctly in a statement, but not an invoice.  

I know how important it is to correctly charge your customer, christine81. I'm here to help you with that.

Yes, you can send an invoice with a credit memo applied. Please make sure that you have only one open invoice for that customer. Otherwise, the credit memo may automatically apply to other transactions.

If you haven't created a credit memo, please follow these steps:

1. Go to + New and then select Credit memo.
2. In the Customer drop-down menu, select the customer's name.
3. Enter the credit memo details.
4. On the Amount column, add the $50 amount. 
5. Click Save and close.

For more details, please see this article: Create and Apply Credit Memos.

Once done, we can now send the invoice with the applied credit memo. Here's how:

1. Go to Get Paid & Pay and then select Invoices.
2. If you have the old interface, go to Sales, then Invoices.
3. Locate the open invoice you'd like to send.
4. Click the drop-down menu under Action, then click Send.
5. Review the invoice, then click Send.
   
   
   

Additionally, I've added an article that'll help you track a refund if your customer wants to have their money back. This keeps your record accurate: Return Customer's Money.

I'm only a post away if you need more help in managing your sales transactions in QuickBooks. It's always my pleasure to help you out again.