How to design relationships for variant data?
This might force a bit of change for your tables, but we do something like this at work:

Ensure each table has a int/guid primary key of some sort:

Organization Table

ID | Name
1  | MyOrganization


Then create a type-table to house your different LeadSource types:

LeadSourceTypeKey | Description
1                 | Organization
2                 | Company Office


Then on your ProductSoldList Table add 2 columns LeadSourceID and LeadSourceType

Then you can easily query the ProductSoldList TABLE and know which LeadSource to join back to:

SELECT *
FROM   [ProductSoldList] PL
       INNER JOIN [Organization] O
       ON PL.LeadSourceID = O.ID AND
          PL.LeadSourceType = 1
       INNER JOIN [Company Office] CO
       ON PL.LeadSourceID = CO.ID AND
          PL.LeadSourceType = 2