Spy agencies have a long history of funding projects through proxies, both government and private, so it’s probably nearly impossible to prove the negative here. The positive remains unproven until there is a leak or declassification.
And in a meaningful way, it might be irrelevant where the money is coming from. The code is open, the papers it is based on are public, the protocol is right there to be inspected. And since it is used by activists and dissidents around the world, it’s been looked at, a lot, by a lot of very smart people.
If NSA wants to fund a tool that is useful, safe, and not-backdoored, I don’t have a problem with that. There are way worse ways for them to spend their insanely huge budget. And if the tool is backdoored, it doesn’t matter where the funding is coming from.
So far, I have not seen a single piece of proof that Tor might be backdoored. If anyone has such a proof, please come forward, as a lot of people at-risk rely on it to stay safe!