Our developer also wanted us to pay the full price back then and promised to refund the penalty payment later. Fortunately, our purchase contract stated that it would be offset against the installment. Does your contract say anything about that? I can understand your bad feeling, I would also be worried about my money.
Unfortunately, nothing like that is in the contract. However, we received a letter from them with the invoice for the 2nd installment asking us not to reduce the invoice and to settle these matters (compensation for delay) mutually with installment 6. We are about to pay installment 5 (if the construction has reached this status), but we have already received an invoice for special requests "tiles" that will be implemented later. That would now be like an advance payment. They add that special requests cannot be offset against the "future compensation claims due to construction delays," as this is excluded from the proceedings (no idea what proceedings they are referring to). The purchase price installments have been assigned to their financier 100%. Furthermore, it states: "Disputes over compensation and guarantees (...) can only be conducted by the developer company after completion of the construction project."
So they are now excluding what they requested back at installment 2 (to settle matters with installment 6) again. Hence this absolutely bad feeling. We will be seeing a lawyer next week, who is already advising us on this matter. My intention was to maybe find like-minded people here. If I am a one-off case here, I am happy for everyone else! Until now, I only knew of such construction delays with projects like the Berlin airport.