$150 is way too much. You can get a small rtc, like 5 inches for like $10 and just grow it out. It will be 2' in a matter of months. These guys grow at an astonishing rate
I believe Op's intent is to rescue an already mistreated RTC, not grow another one out.
I say tell the owner you'll give him 100 cash. I would most certainly bet he takes it...greedy SOB has already sold the same fish multiple times...not like he's losing a dime. It's LFS owners like that that make me want to push for fish to be under the same animal cruelty laws that cats and dogs are...its disgusting to see those giant fish crammed into a small tank...
One of my local LFS has 2 2' rtcs, a 2' rtcxtsn, and a 2' wyckii is the same 125...they lay on top of each other...no room to turn...its horrid...they are all priced at 150 too....if I had a swimming pool and an area big enough to house them...I'd have stolen them months ago...