Can you hold the payments to developer because of delay in the completion?

I am being asked by a developer to pay the remaining 50% of the investment for an apartment by the 15th of Sept., even though the property is still not a full-on construction site (92% according to the developer).

The earliest the building completion certificate will be issued is 1st October. When is the absolute latest I can pay the handover money without being penalized for it and how do I communicate this with the developer.

How much money am I allowed to withhold for the snagging, de-snagging + in case of any handover delays... etc?

My finances are currently very tight and I cannot afford to pay the handover while I am still having to pay rent elsewhere.