Obama can release Trump's tax returns per IRS code

[Santander]

Actually, the section of the IRC they are referring to, IRC § 6103(g), exists partly to prevent abuses of tax return information as was rampant in the Nixon era. It prohibits disclosure or redisclosure to people who are not at least of a certain executive pay grade.

So Obama can request any return from the IRS, but he can't just post it on Facebook.