My house documents like the conveyance deed, lease deed, and possession letter are ready. However, for the allotment letter, no dues certificate, and declaration letter, I only have attested photocopies. Can the buyer cancel the deal based on this? If so, is there anything I can do to prevent it? I need an answer urgently.

In India, the buyer can potentially cancel the deal if the original allotment letter, no dues certificate, and declaration letter are not provided. You should immediately contact the original issuer of these documents to obtain certified copies or originals. You can also attempt to negotiate with the buyer to accept the attested copies while assuring them you will obtain originals soon.