Usually the drain is in a good-sized opening to allow for final fitting, but probably not enough in your situation. You can only adjust the drain as far as the opening in the concrete will allow, or start sawing/chopping concrete. My contractor uses a 5 gallon bucket to create an opening, so I have about 6" of adjustment either way. Depending upon how badly you want the unit you bought, or what the cost would be to return it, modifying the concrete isn't that hard to do if you have the equipment. There isn't enough room between the shower and the slab to do anything else. I have seen a shower built on a raised platform to allow for drain location, but that's not a great solution.

Fresh concrete is not all that hard to cut.