From my experience the best practice for modals is as follows:
Perhaps you have something like fig 1.
Keep modal related messages in the modal box. This includes: validation feedback and save state. Fig 2,3
I suggest changing the button text to "Saving" or similar. This way the message is right where the user clicked and indicates a response to the action. That is good feedback. Be sure to make the button disabled so another save call can't be made.
I put the success response in context of the action. A few reasons?
The modal can be various sizes and the button could be low on the screen. the message will never be in the same space. Also, People expect the modal dialog box to go away when the action is complete. Having it hang a round for a message is like leaving the party too late. People will be busy the wondering why there was no response and won't read your message.
Put the success message on the page, either in context of the saved item or in a consistent and visible spot on the page. Fig 4. This way the users will get the feedback the expect from a saved modal and a consistent feedback message placement.
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