If it's a point-of-sale rebate, you'd have lots of work to make it retro-active on sales that already happened and were settlled.
If you make it a tax credit on income tax, and again try to apply it retroactively into sales during 2021, you'd have people needing to re-calculate and re-file 2021 taxes for the car they bought in late 2021.
All proposed legislation I've seen uses sold-on (delivery) date for qualification, and Jan 1 2022 as the effective date - putting all makers on the same footing, and cleanly making the tax change only hit 2022 tax year.
It would be nice to have retroactive $7.5k clear back as far as possible.... it just doesn't seem at all likely, assuming we get anything at all thru congress.