He didn't realise (like the majority of New Zealander's) that someone other than the registered owner of the property can apply for resource and building consents for a property if there's a Sales and Agreement Form completed even if the sale hasn't been completed..... TA's can't issue the resource or building consent to the purchaser until the sale is completed but they will process them. The developer, using his nuance, got someone else to fund the first part of his project for him.That is not an unusual contract.
Your client could have rejected the offer, he didn't, he was obviously happy with the terms.
He has six months still to live in the house or still have it rented for that period.
Or, he may have already calculated the holding costs ( if any) and capitalised that into the sale price.