MOSCOW (Reuters) -Russia's state oil and gas sales in September are set to fall by around 23% from a year earlier on lower prices and a stronger rouble, Reuters calculations show, hitting Moscow's biggest source of revenue. The projected decline comes as President Vladimir Putin and his economic officials are preparing Russia's 2026 budget while funding its highest military spending since the Cold War. Russia will receive 592 billion roubles ($7.11 billion) in September, according to Reuters calculations based on oil and gas production data, refining, and supplies on domestic and international markets.