Complex sentence: For the 1924 Everest expedition, he was not considered fit as a climber because of his injuries but he was offered the role of base camp and transport officer although he had to turn this down because his employers would not allow him unpaid leave.
Split into simpler sentences: For the 1924 Everest expedition he was not considered able to participate as a climber because of his injuries but he was offered the role of base camp and transport officer. He had to turn this down because his employers would not allow him leave, even unpaid leave.