Giving credits for cost-saving isn't the problem. Companies do that with travel expenses. What you're describing is really the same as giving employees an annual discretionary travel budget.

Here's the problem: It sounds like Google didn't implement financial controls. The employees committed fraudulent transactions by booking travel from outlying years and get credits for use in the current year. That shouldn't have been allowed.

This was unclear in the original post and the commenter graciously clarified.