PO approval between multiple SAP systems

[SPo]

Hi All,

Been looking through a number of forums but couldn't pin point an exact answer or starting point on how to go about this request from the client.

We're implementing a client's new Operational S4 system.
The operational S4 system will report into a Financial S4 system that manages all capital and will be the control and consolidate system.

We want to operationally execute inventory planning with forecasting, replenishment, MRP, etc in the Operational S4 system, including PR and/or PR to PO conversion.

We also want to have a PO approval process based on the usual parameters of $ and Function.

The approvers for the PO's are mainly operating in the Financial S4 system and would like to execute the approval process from their environment - i.e. avoid logging on / monitoring / switching to a different system altogether.

Question is: how can we punch out / connect the Operational S4 system PO's PO approval workflow to be executed from the Financial S4 system.

Is there a way to have PO Approval process /workflow connected between different SAP environments (different system ID's)? 

A workaround for us could be to interface the PO from the Operational S4 system to the Financial S4 system, set up and execute the approval workflow in the Financial S4 system and then interface back to the Operational S4 system when approved / rejected and follow up again.

It just looks a lot more involved, especially for change/rejection processing to work through interfaces than being able to connect a workflow.

Apologies if this is super common, I did have a look, I did see it live in past deployments but couldn't immediately find a "standard" provided way to go about.

Any guidance: greatly appreciated.

thanks!