Ukraine

In theory if Russia wanted to declare a war on the United States, it could probably use "they repaired (and indeed, sent in the first place) these pieces of enemy military equipment" as a justification to their population. It might not be as effective a justification as something like a direct attack from the US on Russian troops would be, but it'd probably be more effective at gaining public support than declaring such a war for no reason and maybe anger uninvolved parties slightly less, so in that sense it could be called a casus belli. However, having a casus belli does not require one to declare a war, and if war is desired, one can probably be fabricated anyway. Russia knows that the consequences of a direct war will the US would be disastrous at best and risk the very existence of their country at worst, depending on how far the conflict escalated, so it will not actually declare such a war over something like this, even if it technically could