Accepted a job with $2000 pay cut from initial negotiated salary. Would it be possible to negotiate that $2000 back?

So long story short, I accepted a job referred by a staffing agency. Initially we negotiated $66000/year, but after some more negotiation, my employer suggested $64000/year, since some "extensive training" with a contractor would be required. I know I probably should've held my ground a bit more, but I was unemployed for quite some time already, and I felt I needed to get back to work and make money ASAP. Fast forward, my employers have been pleased with my work and how fast I've picked up things.

At this point, would it be possible to negotiate my salary back to $66000, especially once the contractor leaves?

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