Urooj Khan made a deal with his business partner to ensure that if he died half of his share of dry cleaning stores would go to his wife, Shabana Ansari, the lawyer said.
Police have questioned Ansari about her husband's death but she has denied any wrongdoing and has not been accused of a crime, the Chicago Tribune reported.
After learning of the agreement, Khan's family members became more suspicious of Ansari than they had been previously, the report said.
The contract with Khan's business partner means that Ansari owns half the business operation and its real estate, valued at $1 million, the Tribune reported, instead of those assets being divided by heirs in court.
Khan's $1 million lottery winnings, amounting to $425,000 in a lump-sum payment after taxes, will likely be split between Ansari and Khan's daughter from a previous marriage, the lawyer said.
The agreement between Khan and his business partner was signed May 2.
Khan died a week after he won the lottery. Testing showed lethal levels of cyanide in his system, the Tribune reported.