What is your Booking Form says?
The lawyer shall draft the SPA base on Booking Form.
Any changes in the booking form must get agreement (with initials) from purchaser n vendor.

Fyi = most agents' booking forms have clause that balance purchase price must be settle by 3 months+1 month.
so now how come the vendor is asking longer than that?
You definately have a firm reason to cancel the deal n demanded 100% earnest deposit refunded back to u

If i were u
- appoint a lawyer to fight for u
- tell the agent as what i said above. Tell him u going to report to BOVEA if u dont refund your booking
- cancel the deal!