Casey’s General Stores Inc. said Thursday that it is negotiating a “transaction” with convenience store giant 7-Eleven Inc. at a price of $40 a share in cash. Casey’s said it remains convinced that the company is worth more than the offered price, but will continue discussions to determine whether a sale can be completed that reflects its true value. 7-Eleven made the offer on Sept. 2. Casey’s believes the company is worth at least $45 a share, and at least one analyst has set the price at $46. Ankeny-based Casey’s rejected an offer of $38.50 per share from Alimentation Couche-Tard Inc. of Laval, Quebec. According to its website, Japanese-owned 7-Eleven’s only Iowa store is located in Bettendorf. The company’s North American headquarters is in Dallas. STORES magazine reported in July that 7-Eleven was the 33^rd-largest retailer in the United States, with Couche-Tard ranked 73rd in its annual list of the 100 largest retailers.