I suspect the loan has been assigned so that your bank does not pay the remaining amount to you but only to the developer. Since the house is not yet finished (free of defects), you cannot initiate the payment. Chicken or egg; cat or tail. Never Ending Story, the developer can sit this out, he knows you are running out of steam.
Are you already repaying or just paying interest?
There is no way around a lawyer. We had about 500 euros in legal fees for the final stretch (it would have been even better to invest 300 euros before signing). For you, it will certainly be more, but it’s no use!
Go to a specialist lawyer for construction law! If you don’t have ~300 euros for a consultation and a letter, then you are already out of the house anyway. Then it doesn’t matter. For everything else: Ask your parents, go into overdraft, take out a personal loan.
Alternative: Declare the house as free of defects ("Basta"), pay the outstanding invoice, and live with it.