The polarizing filter mounts in front of the camera lens and should be optically flat. In other words, both surfaces should be parallel with the same thickness throughout. In many ways, an optically flat filter is more difficult to make than a simple lens. If not optically flat, it will degrade image quality. Luckily, most of the time, the distortions introduced are minuscule.

Most modern DSLR cameras use polarization as part of their built-in auto-focus and exposure metering systems. Because of this, a polarizing filter can interfere with these functions. Cameras that focus using contrast detection are not affected.

To avoid causing problems, there are circular polarizing filters, which are actually two filters sandwiched together. The first is a standard "linear" polarizing filter, which does the polarizing for us. The second filter, called a "retarder", scrambles the polarization so it won't interfere with the camera's autofocus and exposure metering.

I have never had any equipment harmed by a linear polarizing filter, and I doubt you will see any significant difference in image quality. Go for it. Then when your budget allows, get a circular polarizer to replace it.