From just the info you provided, you hold the cards right now until you give them money. I would not give tem a dime until they have fixed this. The seam in the middle of the island will not be going away but workmanship issues are on them.

Don't pay them a dime until they fix it to your satisfaction

Insist that they purchase and utilize a Seam Phantom please.Watch the video and you'll be amazed at the quality of the seams produced. Yours is particuarly bad. Ragged cuts and they screeded the adhesive before it catalyzed so it shrunk below flush. Absolutely amateur stuff and you can tell them I told you. Indefensible.

Of course do not pay the draw.

I'm assuming you have a cantilever on your island? If so, how thick is your stone, how long and deep is the cantilever, and what, if anything, supports it?

It sounds like you're also not happy with the placement of the seam under a light. Did you approve the placement of the seams? We have a similar granite and picked slabs with our specific layout in mind. We also had to pay our fabricators extra to sit-in on the templating of our layout. Money well spent.

This layout could be a much smoother transition if they rework that seam. I wouldn't pay any part of it until it's fixed.