How do I remove or delete a Material line that has been externally requisitioned (purchased and delivered)?

1. Delete delivery (C-P-T-R)
2. Delete lines on purchase order (C-P-T-O)
3. Delete lines in requisition pool C-P-T-Q (these become available again after step 2)

The material lines will now be not marked as requisitioned, so you can requisition them internally or delete them.