Receiving Degree Late due to late payment of tuition
I would put the date shown on your physical (paper) diploma or on your official transcript. If there’s any question about your eligibility or truthfulness, they are what you would show to immigration services. 

That said, most universities won’t give you your final transcript or diploma until after all bills are paid. So in your case, the diploma might show 2006 when it arrives but they won’t actually give you the diploma or transcript to prove it until you pay the bill off.