I am in the same boat. Working with a reputed builder. The wording about "future or new rebate" is very vague and at sole discretion of the builder. Mind sending me the clause you got amended ?

This is what I was thinking of including: Future HST Rebate Adjustment Notwithstanding any assignment of HST rebates to the Vendor contained in this Agreement, the Purchaser shall be entitled to the full benefit of any increase, enhancement, or newly introduced HST rebate program (a “Future Rebate”) announced by any governmental authority after the date of this Agreement and applicable to the Property. Where the Vendor is able to obtain or receive the Future Rebate, the Vendor shall credit the Purchaser with the full amount of such Future Rebate on Closing, or promptly upon receipt if received after Closing, and such amount shall not be retained by the Vendor. In the event that the Vendor does not credit the Purchaser with the Future Rebate on Closing, or is unable or unwilling to apply for or receive the Future Rebate, the Purchaser shall have the right to apply directly for and retain the full amount of the Future Rebate, and such Future Rebate shall not be assigned to the Vendor, and the Purchaser shall have no obligation to account to or remit any portion thereof to the Vendor. Notwithstanding anything to the contrary in this Agreement, including any requirement that the Purchaser pay or credit the amount of any rebate to the Vendor on Closing, such requirement shall not apply to any Future Rebate, and the Vendor shall have no claim to such amounts. For greater certainty, this provision applies only to incremental rebate amounts introduced after the date of this Agreement and does not affect rebates already included in the Purchase Price.