Tell the crew to stop. Call the contractor.

Problem is schedule. New windows will be at least a week away -- sometimes several weeks.

Sliders are generally the low end window, and will look different as well.

Hang on, they are installing sliders and you are supposed to have Single Hung's? How did no one notice the openings were framed incorrectly are sliders are wider than they are all and SH's are vice versa? One doesn't just install the wrong windows, this mistake happened long before installation, what else is wrong? Did you sign off on SH's? Make him change em, of course by letting him finish installing them you may incur his wrath and less the likely hood of his agreeing to change them.

I had this but not as bad of mistake as yours. You tell the contractor to order the right ones. My contractor choose to keep the wrong windows in while they worked because he needed the house closed in. I with held the value of the windows from their next step payment until the right windows were installed. He wasn't happy but that's the right call.