I have seen them before but can't swear to their fit without any smithing being done. It would seem that the barrel bushing on the 9 wouldn't be large enough to accomodate a 40 caliber barrel so I guess the slide would have to be different too. A package with both slides and barrels and magazines for each caliber would easily top $350 around here (Pa.) in good condition.

I'd give Ruger a call and ask them if it were possible. I'm guessing at the very least they would say any warranty is out the door. He may have had to have some work done to make the bbl fit and it may not have come out quite like he wanted it to and he's looking to dump it. A new P89 is pretty reasonable in cost. I definitely would want to shoot it in both calibers before I considered it.

I believe Bar-sto used to make a 9mm for 40 guns but you can't go the other way