Cannot create one to one relationship using look up field

Cannot create one to one relationship using look up field

Hi all 

I am new to zoho and database so i need your guidance

I have one two forms " JOBS " form where each  JOB  has unique job number
and "Invoice" form where each invoice has a unique invoice number
Each job can only relate to only one invoice and vice versa

I have linked by creating look up field in JOBS form so that we can look up for Invoice Number.

The problem i am having is that one invoice number can be selected in many jobs 

Please help how can i relate two forms so that one job is only linked to one invoice