This is a manifest injustice (at least as you have described it). I would simply sue the Landlord for the deposit. Make sure that you have as much documentary evidence as you can muster. Photos, agreement etc. Offer the builder a chance to make a _fair_ offering for cleaning the window. If he won't play fair then sue in the small claims court for the deposit.
The deposit must have been taken before April this year. If it was taken after that date you'll be so glad. Because the Landlord (builder or not) will be in some serious trouble.
Not lodging the deposit with the DPS (say). Not telling you where the deposit was lodged.
The book will be thrown at him - he is just the sort of landlord the law was made to combat.