Samantha,
OK, you have posted the Invoice and sent the Invoice to the Customer. The Invoice, per your original question, was at $110 Unit Price and should be $100 Unit Price. You asked how to correct this so it is reflected in the Profit of the Order originally posted, which has a Unit Cost. The only way to have this Price Change effect the Gross Profit of the Sales is to Correct the Original Posted Sales Invoice and repost the correct Unit Price. Here is an example of one:
I have the below Posted Invoice and the Unit Price should be $85.00. I go to the Posted Sales Invoice and click Correct/Correct. This will cancel the Original Invoice by creating a credit Memo. The process creates a NEW Sales Invoice with the same information as Posted Sales Invoice 103045. I then update the correct Unit Price. Below are all the screen shots.


I posted the Sales Invoice and my Gross Profit is updated against the Order.
Hope this helps.
Thanks,
Steve