Customer wanted a TV cabinet measuring inside 40" x 17". Before I have
gone too far she tells me she mismeasured the TV; it needs to be 40" x 20".

Deliver it; turns out the TV is 20.2" high. Argh.

Well, the bottom piece is butt jointed between the sides, so in principle I
could remove the bottom and put a new bottom below the sides. That would
gain 7/8". It would be less than desired, but at least it would fit.

The bottom is fastened in with pocket screws and Titebond Molding & Trim
glue; I wanted something that wouldn't run while I was putting the bottom
in.
Any way to unglue it? I can remove the screws and saw the bottom off but it
would be better to unglue it if possible.

Yeah, I should have measured it myself, but she is 45 miles away. Yeah,
driving 90 miles would have been better than this.

The bottom is fastened in with pocket screws and Titebond Molding & Trim
glue ....

Any way to unglue it? ...

Yes. Use a heat gun to soften the glue. I had a similar situation with
Titebond III. The usual way to loosen the glue joint is with white
vinegar, but, since Titebond III is waterproof, I couldn't do that. I
called the manufacturer and talked to a tech rep. He suggested heat and
it worked perfectly. I was able to use a reversed clamp as a spreader
to force the soft joint apart. You may have to tap it with a dead blow
mallet. Good luck.

Joel Jacobson