How do you link the customer to the supplier?

How do you link the customer to the supplier?
Because it is possible to exchange goods between them
The job I do, there is an exchange of goods between our company and another company

You do not. Read the Guide: Offset simultaneous sales and purchase invoices | Manager.

Note, however, that in accounting, your business is not considered either a customer or a supplier. Both of those terms refer to other entities.

Manager does not currently support that internally. Several threads have discussed trying to support it, such as this recent thread Customer/supplier reconciliation reports - #19 by Patch