The stimulus payment is based on your 2020 status, which there is no way possible for the IRS to know definitively until you file your 2020 tax return next year. The IRS just used the 2018 and 2019 tax returns as guidance, so as to get "something" out there to folks.
When you file your 2020 tax return next year, if you qualify to claim the dependent on your 2020 tax return and you actually "do" claim the dependent, then the treasury department will "settle up" with you at that time.
