For 700k property buying via the secondary market (subsale), it doesn't matter actually before or after Jun. Now or end of year is the same.
You wont be receiving any stamp duty waiver at all.

For property priced above 500k, the government initiative is to help developer clear out their unsold stock in hand / developer units. If you are getting such property direct from developer, then only will have some incentive from the stamp duty waiver.

Yeah same. Many doing the same early this year. Back dated the signing to Dec 18. Coz stamp duty waiver for below 500k subsale property is still valid until 31 Dec 18.