There is no published decision date to wait for, so treating approval as a scheduling anchor invites disappointment. Sequence the purchase around what is knowable: when funds documentation is complete, when the enterprise accepts subscription, and when a visa number would realistically be available for the applicant's chargeability, which usually follows country of birth.
Anchor dates to milestones you control
Useful markers include a completed source-of-funds file, signed enterprise documents, the petition receipt, current Visa Bulletin movement for the relevant category and chargeability, and the consular or adjustment stage. Conditional residence follows, with the petition to remove conditions ordinarily filed within the ninety days before the second anniversary. A property completion clause that depends on any of these should be negotiated with a lawyer, not assumed.
Negotiate the property transaction so that nothing in it depends on an immigration outcome, since a completion clause tied to an approval is a clause tied to a date nobody can supply. Where flexibility is needed, ask for a longer completion period or a right to extend on defined terms rather than a condition referencing a decision. Anchor the immigration side to the milestones the household can observe: the source-of-funds file complete, enterprise documents signed, the petition receipted, and movement in the relevant category and chargeability.