Is there a way to monitor the material at the subcontractor?
Yes depending upon your requirements and processes, but monitor is a subjective word - you would need to tell us about your processes and requirements.
Send a sub-contacting purchase order with material to a vendor for some work. Want to know what material is where?
Transfer the material to a vendor warehouse, consume from the vendor warehouse at the point the purchase order for the service is received to indicate the service is undertajen and the material has passed through the value add process.