Finance corrects a billing address in the ERP. At night, a CRM export replaces it with the old address. Neither transfer failed. The connection accepted a value from the wrong source.
The rule belongs to the field
Sales may maintain a contact while finance maintains billing details. An instruction for one delivery belongs to its order. Assigning the entire customer record to one system misses those distinctions.
For every shared field, record its meaning, accepted source and update direction. Also decide whether an empty value means unknown, unchanged or deliberately cleared.
Test two edits before the next transfer
Change the same field in both systems, then run the transfer. Check which value survives and whether a conflicting correction needs a person to review it.
The shared field register records these rules. Keep it with the mapping and tests so a later change does not silently reverse them.