Hi,
I am not sure I get it - could you describe a bit more the scenario of your customer and what you want to actually achieve, please?
There is no single Badi, which would call the other system. The CRM middleware is a robust framework, which has lot of content and configurations. In standard setup, data between systems are not exchanged synchronously but asynchronously, using qRFC (queued RFC). Even if your business flow is one-way, technically there should be two-way communication setup.
Example: you create sales order with single line item in CRM, even without pricing. The product is defined as BOM in ECC. Now once you save the transaction in CRM, a BDoc is created and placed into an outbound queue (transaction SMQ1). Once the outbound scheduler triggers the queue, the BDoc is transferred via RFC to ECC. In ECC, the BOM is exploded, thus more line items in the order are created, and priced. ECC immediately creates a validaction BDoc, which is sent back to CRM (again asynchronously) to update the CRM sales order. If the user changes status on the sales order in ECC, it will send information back to CRM.
The transfer from CRM to ECC is controlled (among others) by Administration console (transaction SMOEAC in CRM), whereas transfer from ECC to CRM is controlled by table CRMRFCPAR (in ECC).
There are many settings, generated function modules, and also BAdis delivered - however, the latter are usually used to fine-tune replication scenarios (maintain fields content or prevent replication).
Regards,
Dawood.