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The end date of the Soviet-Japanese Border War oddly coincides with the beginning date of the Soviet invasion on Poland in 1939. Was the end date of the Soviet-Japanese Border War the direct reason of starting the Soviet invasion on Poland on 17th of September 1939?
(In other words if the Japanese army had stood longer, then the Soviet invasion on Poland would have been delayed and might have had an influence on how the Invasion on Poland moved.)
The question has it backwards: It was the planned invasion of Poland that caused the peace agreement to happen, not visa versa.
This war was never officially declared, and it took place on the border of Mongolia and Manchukuo (a Japanese puppet state) because of the disagreement about the location of the border. Militarily, the Soviets (and Mongolians) prevailed. But they had to agree on cease fire because their planned invasion to Poland was more important than the Mongolian - Manchukuo border dispute. The ceasefire was signed 2 days before the attack on Poland.
The Molotov Ribbentrop pact, which made the invasion of Poland possible and feasible was signed on Aug 23, 1939, while the decisive Soviet offence in Mongolia began on August 20. So the Soviets understood that they were winning in Mongolia at the time the decision to invade Poland was made. This Soviet-German pact was a sudden opportunity for the Soviets, and it happened in the middle of a less important conflict in Mongolia. So they did everything to end this minor conflict.
In April 13, 1941 a Soviet-Japanese neutrality pact was signed (it left the question of Mongolian-Manchukuo border open).
The agreement on the border was only signed in May 1942, and Soviets essentially yielded to the Japanese demands (despite their military victory). The reason is of course that Soviets had much more important things to care about in 1942.
I conclude that the causal relation between the attack on Poland and Soviet-Japanese ceasefire was just the opposite to your suggestion. The Soviets had to sign a ceasefire, because they were in position to do this (they prevalied militarily), and because the invasion of Poland had much higher priority.
(The dates of the treaties are based on Russian Wikipedia).
Remark: The Soviets were fighting the Kuantung army in this war. The Kuantung Army was formally a part of the Japanese Imperial forces, however it was not completely controlled by the central government. According to the (English) Wikipedia, the whole creation of the Manchukuo state was an initiative of the Kwantung army, not of the Japanese government. Eventually the Japanese government approved it.
