Sync Async scenario with a twist... Possible?

Hello experts,

I have a requirement - Depending on the message processing status of the incoming message, SAP PO needs to send back a technical acknowledgement back to the sender.

REST <-> SAP PO <-> SAP [Proxy]

1. A request comes in Synchronously via REST sender channel.

2. The data is sent to the back-end SAP system via Proxy

3. If Step 2 was successful - send 200 in the XML body back to the sender, if Step 2 failed - send 500 in the XML body back to the sender.

I need to achieve this without a BPM or Custom Modules.

Any ideas?

Thanks!