I would skip the auto-validation and let the user hit the always-enabled "Confirm Donation of $xx" button to validate the form. Tried and true and catches the auto-fill cases.
Yeah, we'll consider that. Thanks Dave! ;-)
A lazy solution is to periodically check for changes to the field, independently of any events. Could also let the user behavior trigger a validation check.
I wonder if a focus/blur combo would trigger the needed event and the value would be set? I need to try it out.
If you're using the jQuery Rails validations I believe you'll have to set the input's "changed" data to true before it'll run the validations again. Something like $(".my-input").data("changed", true);