Create two condition type
ZCAH and ZCAS
Set up ZCAH condition type as positive and ZCAS for negative.
Then in calculation schema, enter the ZCAH and then ZCAS condition type
Next enter the total and give the "from and to" as bellow screen.
Then whenever you will give any amount in condition type ZCAH and ZCAS, the total will appear as ZCAH-ZCAS.
For more details please read the blog Pricing procedure Steps and Details in SAP MM
Regards