WOW!
I feel your pain.

I would check out the bear and recheck his measurements. If through visual and mathmatical inspection you are still not sure then the bear you are paying for is more than likely not yours. Chances are your bear was lost, sold, or had problems like hair slipping, torn during tanning, etc., and the taxidermist does not want to tell you the truth or lose your deposit.
Sound like he is shafting you all the way around. 5' 3" rounds down to 5" in my book and there is almost no way a bear of that weight would come to 5' unless it was an older bear in severe dietary distress.

Years ago I had a great taxidermist who did a bunch of deer and bear for me and my family in a timely fashion. One year he took twice as long to do a couple of bear mounts for me and my brother and when we got them back I was positive that neither one where bear we killed. I packed out both from the woods and the mounts seemed to be larger bear. I was skeptical, but continued using him on animals that I could identify by colors, size, etc.
Well a couple years later I spoke with an old family friend whom was a bear hunter and happend to do taxidermy on the side. He told us that our guy "got in trouble" and "lost some bear" and had called him wanting a half dozen bear hides. The family friend sent him three and I assume he got the others elsewhere.
But on the other hand the family friend will really stretch the bear rugs he does 'cause the clients want "big" bears.

The mount is on my wall, but it will be the first to go if I need room as I don't think it is mine.

My suggestion is this.
Call other taxidermist' within a couple hundred miles of him and ask if he has purchased any bear hides from them and ask their opinion of your guy.
If you are 100% it is not your bear DON'T pay for it. You will always wonder and probably kick yourself everytime you look at it. Hopefully you got some good pictures and can make some sort collage as trophy.

Keep us posted!