I hope you have created the service line item (in PR) by selecting the option Service Selection and then select From Purchasing Document as Contract.
As of my knowledge, there are no report or t-code where you can find the relation.
If you want to take it from table level, then one relation is there for package number.
Go to EBAN table enter the PR number and take the value for field PACKNO,
Then go to the table ESLL, pass the value of PACKNO +1 in field PACKNO (+1 means as I get from EBAN table as 2848, then I need to enter the value as 2848+1 = 2849 in field ESLL-PACKNO)
Then take the value for field PLN_PACKNO from this table.
Then go to EKPO table, enter the value of PLN_PACKNO -1 in field EKPO - PACKNO (-1 means 2569-1 = 2568)
Here is the contract number as EBELN value.
Try it .. If you want it as a report, then please tell the logic to your ABAPer, he will make a report for it.