Reiterating the answer given by user Silverfish, you should not short pay your balance due regardless of whether you received the full stimulus payment or not. The stimulus payment should be resolved on your 2020 tax return and would not be tied to your 2019 balance due.
Underpaying the balance due will result in penalties and interest. Also, filing an extension does not help with regard to a balance due. The extension is extra time to file your return, not extra time to pay.
@barnsablazin
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