I think not many realized that you were talking about a job change during a school year. At least, that's what I think you're indicating. That might explain why District B won't talk salary unless they know that District A (your current employer) is willing to release you mid-year. And that may be different than actually "being released", i.e., you have resigned and released from your contract with District A.

About 20 years ago I want through this very process. District B offered me a position at the beginning of the next school year, then another position at the same district unexpectedly opened up mid year. I was able to make the mid year switch only because District A allowed me to be released from my contract with them mid-year. Maybe this permission to be released is what your District B wants to have in place before they talk salary with you.

As an aside, know that if you make a mid year switch, your salary and paychecks mayl be wonky until the start of your first full year with District B. District A may pay out the remainder of your stipend/extra days while the same will be very small for District B until the new year starts ... this is what happened in my case.

Hope this helps!
Two Sheds