Do I create a Sales or Purchase adjustment?

If it is for the Supplier: Purchase Adjustment. You can either do a debit or credit, depending if you need to pay them more (i.e. for a heavy weight from the Customer) or reduce what you have already paid them (i.e. for light load or material downgrade). Just put a negative in the amount field and it will change the type of memo.

If it is for the Customer: Sales Adjustment. Again, you can either do a debit or credit, depending if you need to pay them (i.e. a light load or material downgrade or partial/full rejection) or charge them (i.e. heavyweight or material/grade change increase).