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Both the systems in question has have the ability to specify custom extra fields in the customer screens which could be used to store the IDs in of the other system.
(Hint this is core integration concept - ID mapping)
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The people with the knowledge were sales staff and the finance team. Given we were talking about thousands rather than millions of records, the easiest way to solve this was to use Iguana to make a data feed from both systems and combine the output into an excel spreadsheet. To do this the integration engineer had to write a couple of simple adapters for each system.
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The key here is to quickly deliver something useful after getting the alignment done. If you make something useful that makes the job of the sales people and the finance staff easier, then you have created the motivation for both teams to put the effort in to keep alignment of the systems.
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