Can I split billing across multiple departments or cost centers under one CSP account?

Yes, to an extent — Azure supports tagging resources and using management groups/subscriptions to separate costs by department, project, or cost center, which works the same under a CSP Azure Plan as it does directly. For Microsoft 365 and other seat-based licensing, some partners can also provide cost-center-level reporting even within a single consolidated invoice.

If internal cost allocation is important to you, it's worth setting up a consistent tagging/structuring strategy early, and asking your partner what reporting they can layer on top of that.