I wonder if anyone here knows @US IRS?

[Taem]

I received a notice that the US Gov overpaid me and wants to collect this over-payment. They are threatening to charge me interest on this money, despite it not being a loan, a debt I owe, a lean, or promissory note. That's not what concerns me however... if this is legal, then what's to stop me from sending someone $10K in a check, waiting until after the check is cashed, then demanding repayment with 100% markup interest saying I sent it to the wrong person so I'm charging interest? How is it legal for the IRS to do this? If it is legal, then what's to stop others from doing this as well.

The money the IRS collects in interest... it's technically income and should be taxable as well, right? Who collects the taxes the IRS pays on interest income? If they aren't paying taxes on their income earned outside of strictly tax collection (i.e. interest earned), then this is technically illegal, correct? They would be breaking their own rules on taxing income.

For whatever wacky reason, I have yet to receive my stimulus check. I've read I will receive it when I do my taxes. Am I not entitled to charge the IRS the same interest they plan to levy against me in my first example for non-payment? If not, this seems incredibly contrive and unjust. Is the system really that broken?

[Lissa]

Have you bothered to look up the IRS number from the IRS website (IRS.gov) and talk to them? Considering the number of scams out there, I would call and confirm.

Also, if you made over 100k in 2019, you won't get a stimulus check (as someone who did and didn't get a check).

[GhastMaster]

It is likely that from the viewpoint of the IRS, you erred calculating what you owe them and therefore were not entitled to the overpayment. Hence, you owe interest on the wrongfully gained monies.

...then what's to stop me from sending someone $10K in a check, waiting until after the check is cashed, then demanding repayment with 100% markup interest saying I sent it to the wrong person so I'm charging interest?

Civil courts could decide in your favor.

The money the IRS collects in interest... it's technically income and should be taxable as well, right? Who collects the taxes the IRS pays on interest income? If they aren't paying taxes on their income earned outside of strictly tax collection (i.e. interest earned), then this is technically illegal, correct? They would be breaking their own rules on taxing income.

The IRS is a "non profit". They do not pay taxes.

For whatever wacky reason, I have yet to receive my stimulus check. I've read I will receive it when I do my taxes. Am I not entitled to charge the IRS the same interest they plan to levy against me in my first example for non-payment? If not, this seems incredibly contrive and unjust. Is the system really that broken?

From the IRS:

By law, the interest rate on both overpayment and underpayment of tax is adjusted quarterly. The interest rate for the second quarter, ending on June 30, 2020, is 5% per year, compounded daily. The interest rate for the third quarter, ending September 30, 2020, is 3% per year, compounded daily.

Consult an accountant/attorney!