Easy.

Save the sha256 hash of original.dat in compressed.dat. The decompressor cats /dev/random until data of the right size comes out with the correct hash.

Now there are two cases.

1. The reconstructed data is actually equal to original.dat. Challenge won, cash in $5000.

2. The reconstructed data differs from original.dat. It has the same hash though, so you found a collision in sha256. World fame.

In either case, win!