Hi Ravi,
I'm not sure if there's a user-exit when the PR is cancelled. But for PR create, you can consider the following Enhancements and see which one, if at ll, is suitable for your case -
SAPLMEWQ : Customer exit for processing of requisitions via BAPIs
MEREQ001 Customers' Own Data in Purchase Requisition
Regards,
Anand Mandalika
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