I believe the basis for releasing Donnie's tax return is 26 U.S. Code § 6103 (k) (1)

[Jack the Greater]

... which states, "Return information shall be disclosed to members of the general public to the extent necessary to permit inspection of any accepted offer-in-compromise under section 7122 relating to the liability for a tax imposed by this title."

An "offer-in-compromise under section 7122" is an agreement between the IRS and a taxpayer to settle a tax debt for less than the amount owed. When this happens, the general public is granted access to the return upon which that agreement was based. I imagine that this access is granted to the general public because someone is being "let off the hook" so to speak, for a tax liability, and the general public has a right to know what this agreement was based on. That seems reasonable to me, as the general public has to to make up the difference between what was owed and what was paid.

[FBaggins]

1. How could it be?

Congress can’t make an offer for the IRS - and TFG obviously hasn’t accepted such an offer.

[inthewind21]

2. How is it

Obvious?

[Jack the Greater]

3. The way this makes sense is that Donnie triggered the release to the general public...

... by accepting an "offer-in-compromise under section 7122" at some time in the past. Time will tell. There is some early chatter around the internet that 26 U.S. Code § 6103 is the basis for releasing Donnie's tax return. After examining this section, I found the kernel in 26 U.S. Code § 6103 (k) (1). It is the only part of this section that deals with release of a taxpayer's return to the general public. By accepting a deal to settle the tax debt for less than what is owed, the taxpayer who accepts relinquishes the right to privacy for that return upon which the deal was based. Congress has nothing to do with it. It is the law.

[FBaggins]

4. Except it really doesn't

The IRS lost a lawsuit last year over this very issue. EPIC won an order in court requiring the IRS to turn over any offers in compromise involving Trump.

A few months later, the IRS completed the records search and came up empty. So there presumably are no OICs to fit your imagined scenario.

Lastly - that section would only permit the release of the information involved in the offer/acceptance itself. It wouldn't involve years of full returns.

Information tied to any compromise agreements that former President Donald Trump made with the IRS over his personal and business tax returns must be released, even though the underlying returns are protected, a federal court ruled.

https://news.bloombergtax.com/daily-tax-report/trump-agreements-with-irs-but-not-tax-returns-must-be-released