Tracing the data flow across Distributed/Legacy systems

I have 2 Microservices [A, B] and the service [B] has integration with legacy system [C]. The service [B] usually generates unique-identifier and includes it in the flow into [C], also passes it back to [A]. In that way reconciliation was handled between the systems.

The problem really started when there is a need for a service [D] that [A] has to invoke in parallel to inform [C].

While it makes some sense to have [A] generate the unique-identifier and send it to both [B] and [D] to solve the problem, its not that easy because of the cost of change in [C]. So we still somehow need to have the [B]’s unique identifier available for [D] to marry the data in [C].

Appreciate if someone can guide me if there is tactical pattern to solve this problem.