How can I associate an A/P invoice to a service call?

Hi,

Customer wants this:

1. They (Co. A) gets a service call from their customer say for lump sum of 400 dollars.

2. They (Co. A) give out a contract to technician say at 45 dollars an hours and travel expenses. This is SERVICE ITEM for a total expense of say 45*2+120=210.

3. Technician (A Vendor BP in SAP Business one) produces a PO. Co. A pays off the Technician.

How can I create a Service Call to show both ends of incoming revenue and outgoing expense in the service call?

Appreciate your help.

Thank you.