You're not thinking fourth-dimensionally.
It works the same way that plot devices function in Bill And Ted's Excellent Adventure. You need cash? Just remind yourself in the future to send yourself back some. Given that the project leads to creation of the most popular gaming accessory of the 21st century, a mere few thousand dollars is next to nothing. Unfortunately, since the accessory effectively put all software developers out of work and marked the beginning of the collapse of worldwide economics, currency of the future is also worth nothing.
Indeed...the meek inherited the Earth.