What should I ask before having a sagging beam jacked up?

First, make sure the recommendation is based on seeing what the beam actually is and how it’s supported. If the ceiling needs a small access opening to confirm its size, condition, connections, and bearing points, that seems worth discussing. Ask for a written plan that covers the beam, the new post, and what will carry the post’s load below.

A basement slab may not be designed for a concentrated load. The contractor or engineer should verify whether there’s an adequate footing already, or specify a new one, rather than assuming the slab can take it. I’d also ask whether the piano’s location and weight were included in their assessment.

Raising an old beam isn’t just a matter of turning a jack until the floor looks level. The floor, walls, and finishes above may move too, and doors or trim can end up out of alignment. Ask how much they propose to lift, how they’ll do it gradually, and what signs would make them stop.

Also ask what result is realistic. A beam can have a permanent bend, so adding support may improve stiffness without making the floor perfectly level. If the main goal is a steadier-feeling floor, say that plainly so the plan matches what you care about.

I’d separate the two decisions: whether the beam needs extra support, and whether it should be raised. A properly designed post might support it in its current position, while lifting it could cause more disruption and still not remove all the slope. Get the proposed work and expected outcome spelled out before agreeing to both.

If the engineer didn’t inspect the beam directly, or you’re not comfortable with the assessment, a second structural opinion is reasonable. Ask for a clear explanation of the load path and foundation details, not just a quick suggestion to put a post under the middle.