Subcontracting or service Po

Hi,

We are sending machines to a vendor for repair. Most of the times we are providing the spares- say as an example gear to the vendor. Sometimes vendor can also use this gear from his own if it is not available with us. In addition if required some other spare in the machine need to be changed, he can use his own spare.

He sends us the bills for his services and of spares if he has used any.

This scenario will fit in which process- Subcontracting or service Po?

If it is service PO than how can we issue the material to the vendor and how the accounting entries will look like?

Regards

Neeru