1, isn't that the purpose of asking you to borrow 30 thousand more? You can borrow some more money if you don't want to.
And you can sign a supplementary agreement with him. Let him explain what you are worried about.
When the house comes down, the loan has already come down, otherwise you can't buy a house. The tax has been paid, and this cannot be amended.
3, you have to pay in full, and the purchase contract can't be modified generally. Your developer didn't check your credit information before buying a house. How much can you borrow? How do you repay the loan now?