That would be an easy one for me to decide. Install the one that came with it and get the free one as soon as it comes in.

You now have a spare liner to use sometime down the road. Here are a few things that could happen.

The chlorinater could spill and leave ugly stains on the bottom of the pool. Someone careless with a vacuum could poke holes around the outer edge of the pool. There are many things that could happen. You have a free liner to learn and experiment with, see what works and what doesn't. If you have a sand base the sand will compact better over time and will stay smoother the second time around.

I think you have a great situation, even though it is not exactly what you wanted.

Dennis