Soldering a Tee onto a Copper Pipe That Only Moves About 1cm?
If you make another cut in the pipe (either above or below your T - it doesn't matter), you could fit a Union there:

Solder the 2 halves of the union first so that they're ready to re-join the new cut, then solder the T.
Since one piece of the pipe you're soldering into the T only has the half of the union on it, you should have enough wiggle-room to get it all assembled.
Once you've finished soldering the T, screw the union together.