Scenario: You have an Event where the registrant is allowed to specify their own fee/donation upfront (during registration) but you also provide them with the option to be invoiced for that specified amount later (the 'Invoice Me' option). How can you know what amount the registrant entered (and expects to be invoiced for)?
Once the 'Invoice Me' option is chosen, the system does not record the value manually-entered into the 'Other' amount field. So, there's nowhere to go look it up in Salsa. That said, you can use auto-responder merge fields to accomplish your goal. You can set up an auto-responder that notifies you of registrations *and* includes the amount (Event_Amount). Here's the documentation for obtaining those additional merge fields:
And here's the basic documentation for setting up auto-responses, in case you haven't set one up before: