Wal-Mart Stores Inc. (WMT) said Friday it had asked former Vice Chairman Thomas Coughlin to resign as a director in connection with an internal investigation into unauthorized use of corporate-owned gift cards and personal reimbursements.

Coughlin, who sat on three board committees, including the one for strategic planning and finance, resigned from the board Friday in a disagreement with the company over the matter, Wal-Mart said in a filing with the Securities and Exchange Commission (search).

Wal-Mart said it had reported the matter to the Attorney for the Western District of Arkansas (search) based on the findings of the investigation, including Coughlin's response to questions about his knowledge of certain transactions.

The world's largest retailer also said it had fired three employees, including one officer, over the investigation. A company spokeswoman declined to name the employees.

The amount involved is estimated at between $100,000 and $500,000, Wal-Mart said.

"The investigation relates to personal reimbursements, payment of third-party invoices and the use of company gift cards," Wal-Mart said in a separate statement. "There will be no adverse financial impact to the company in connection with this matter."

Wal-Mart said in December that Coughlin would retire the following month as vice chairman after 25 years with the company, but would remain on the board until its annual shareholders meeting on June 3.