Hi Keith Taylor,
You may try with "Net Procedure" Function to calculate discount when posting an outgoing payment to vendor.
1. Select "Net Procedure" under AP invoice--->Accounting Tab
2. Define cash discount under payment terms
When you raise outgoing payment, the cash discount field is updated with 2 %. Refer screen shot of outgoing payment screen:
For complete details on cash discount setup, you can refer "How to define cash discounts" guide from SAP Business one customer portal.
Thanks & Regards,
Nagarajan