Credit note from Job worker

We have job work charges outstanding as 1,00,000 and job worker sold scrap on our behalf worth 18000 + gst and has the amount. 

Can we raise debit note and reduce ac payable by 18000 in name of job worker against 1,00,000 job charge? Is it valid?

Or 

Do I need to raise invoice for sale of scrap to job worker?

Thanks.