How to configure AP Invoice approval workflow based on Budget Availability of Expense

Summary: We need to configure AP Invoice approval based on the condition if Budget is available or not.

Content (please ensure you mask any confidential information): We have requirement where Invoice Approval Amount conditions are different when budget is defined & available V/s budget not defined or not available for any expense. For example, if the expenditure is within budget & the invoice amount is more than 5 million $ than it should go to Group CEO, however if the expenditure is not within budget or no budget is defined & amount is more than 5 million $ than it should go to Group COO.

Kindly suggest how to achieve this requirement.