How to throw custom error message when user selects wrong value of "Transaction Type" field

Summary:

How to throw a custom error message when the user selects the wrong value of the "Transaction Type" field which is populated based on the Transaction Source Field value in the Create Transaction page in Receivables → Billing .

Below I have given a business case scenario requested from the business:
In the Create Transaction Page, if business user selects Transaction Source as "ORA_Subscriptions" then the Transaction Type should be "DSI_OSS_Invoice". If the user selects any other transaction type for Transaction Source is "ORA_Subscriptions" then custom validation error message needs to be shown "Wrong transaction type selected" while clicking on Save and Close button or as soon as the Transaction Type field is populated.