June 11 (Reuters) - Debt-laden French tech group Atos said on Tuesday it has entered exclusive negotiations for selling its Worldgrid unit for an enterprise value of 270 million euros ($289.93 million) to engineering consulting group Alten. ($1 = 0.9313 euros) (Reporting by Shivani Tanna in Bengaluru; Editing by Shounak Dasgupta)