I see three options for you.
1. Change the foreign key so that it uses the same columns for the parent and the child.
2. Add computed columns to the table so that if the Lead_Client value is not null then the values are entered in automatically.
3. Change the front end so that the only people who show up in the Lead_Client drop down list are the ones who have the same Code and Date.
Option #3 would be the best option as this will prevent the users from selecting incorrect leads (which they will if you let them).