Why you need to do it via user exit or badi ?
You can set the default value during contract creation.
1. You can set the field value defaulted from vendor master data
In XK02 - Purchasing Data tab, you can set the Pricing Date Control
2. If you want to set the field value for a combination of vendor and material, then you can use info record.
If you requirement is different, then you can tell your requirement.