This is a relatively long article on how the Poland/MIG/F-16 deal fell through, but well-written: https://www.politico.com/news/2022/03/10/poland-fighter-jet-deal-ukraine-russia-00016038
sorta tl;/dr:
1) Ukraine pleading for more aircraft
2) Negotiations began between Poles (when they offered a deal) and a few other NATO countries.
3) But, EU foreign affairs sec./foreign policy chief Josep Borrell opened mouth 2/28 and said, "we got us a DEAL here." But it wasn't yet.**
4) Ukraine ran with it, partly out of hope, partly "cat's outta the bag" pressure to provide.
5) Poland, Slovakia and Romania (all sources for the MIG's) knocked it down and cited that they won't enter Ukrainian airspace to deliver them 3/2.
6) Enter U.S. with that complex "MIG's to Ukraine, F-16's to Poland" potential deal 3/3.
7) Deal seemed to have legs, but then quickly halted, mostly due to U.S. military experts' fears. (F-16's WAY too high-tech even for Poland and they didn't want them having that tech so close to Russia, plus the biggie - supplying a NATO country with these to be taken into Ukraine = ramping up to war.)
8 ) U.S. tells Poland though, go ahead and send MIGS if u want... however the F-16's we'll give you will take months to deliver (needs a lot of vetting).
9) Poland a bit irritated, then offered the 28 MIG's to Rammstein, where Ukrainian pilots would travel, pick up, and fly into Ukraine.
10) U.S. caught by surprise for that, still said too much U.S. involvement (use of U.S. base) = ramping up, plus Scholz said, "no, not in Germany you won't").
11) U.S. military experts said nix it, Poles were out of order, and it's too much of an escalation. Encouraged Poles to deliver as they might/could.
12) Tensions got a little tense, Harris sent over to kind of smooth things out. ** It's worth noting that a deal might have been doable in secret ("how did those MIG's get there?") if Borrell didn't shoot his mouth off too quickly.