Make a simple card template of the piece of glass and go to your local glass merchant and get them to cut a piece.
Then go and fit it in - secure it with a couple of panel pins and tell them it should have had some beading on the inside to keep it in place.
Shouldn't cost you more than a fiver
Don't waive your window cleaning fee - you've done the work.
I used to break the odd pane when I was trad (maybe one every other year or so
) Even if it wasn't my fault I would always replace it, simple good customer relations. It never cost much, just a bit of time. I even had a stock of glass so I could cut simple rectangles myself.