There is an anti avoidance provision under the RPGT Act, i.e. s.25(2):-

(2) The Director General, where he has reason to believe that
any transaction has the direct or indirect effect of—
(a) altering the incidence of tax which is payable or suffered
by or which would otherwise have been payable or suffered
by any person;
(b) relieving any person from any liability which has arisen
or which would otherwise have arisen to pay tax or to
make a return;
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© evading or avoiding any duty or liability which is imposed
or would otherwise have been imposed on any person by
this Act; or
(d) hindering or preventing the operation of this Act in any
respect,
may, without prejudice to such validity as it may have in any other
respect or for any other purpose, disregard or vary the transaction
and make such assessments as he considers just and proper in the
circumstances.
(3) In this section “transaction” means any trust, grant, covenant,
agreement, arrangement or other disposition or transaction made
or entered into (whether before or after the commencement of this
Act), and includes a transaction entered into by two or more persons
with another person.

If the DG has reason to believe that you are evading tax on purpose, they can calculate your RPGT payable based on their assessment and disregard your selling price. For example, in your case, if they have reason to believe that you are doing so to evade tax, they can calculate say based on the market value of the property and disregard the selling price stated in the SPA.