Supplier-customer relationship between different operating units

We have a scenario where there
are two operating units involved. E.g. the operating unit 1 is built for company
X and operating unit 2 is built for company Y. in reality the company Y is a supplier
for company X i.e. company X will be the customer for company Y.

Now for one service provided by
company Y we need to charge the amount to company X.

Kindly suggest what’s the
difference between creating a payables invoice in operating unit 1 against the
supplier company Y and creating a receivables invoice in operating unit 2 against
the customer company X.