Starting from past release of 5.3 (version 5.30.1966) Acumatica has added new option to help you to define different approval logic depend on the budget validation.
On the RQRequest DAC now we have a new field "Is Over Budget", that you can check when you configure your approval maps.
This flag will be automatically calculated by the system, when any line in the request has exceeded the configured budget. Actually this flag represents the budget validation warning.
The budget validation and system behavior on budget exceeding can be configured on Request classes form. System can raise errors, warnings or do nothing. If we want to approve over-budget items manually this should be a warning, as in the error case Acumatica will not allow to release request even if it is approved.
Have a nice configuration!