@re4mul8
Unfortunately, you did constructively receive the missing 20% since it was sent to the IRS in your name and is to your benefit, to offset any taxes and possibly be refunded.
(Do you have a 401k or 403b? If that plan offers loans, you may be able to borrow up to $50,000 in just a few days with very little paperwork.)
Also, even if you can't find the whole 20%, any amount that you rollover in the 60 day window will help reduce the taxes and keep more of your money in the IRA for your retirement.
Also note that if you are under age 59-1/2, there is an additional 10% penalty for early withdrawal (unless you are also permanently disabled). I did not mention this before because I assumed that, if you have access to your pension as a lump sum, you are either past that age or disabled so the 10% penalty won't apply. But it might be important for other readers.
If you look at the revenue procedure linked above, you will see a list of reasons and a model letter you can use to self-certify an extension to the 60 day rollover window. Note that the IRA custodian is not required to accept your self-certification. So if you want to try and borrow the missing 20% and deposit it as a late rollover, you should ask them first if they will accept it. If they will, then you have to decide for yourself whether your situation meets any of the allowable circumstances. If audited, the penalties will be on you, not the IRA custodian.
Bear in mind that the normal statute of limitations for audit is 3 years. So if this happened in 2025, the IRS would have until April 15, 2029, to audit your return for failing to treat the money as a withdrawal and paying income tax. However, if you put the 20% back in the IRA and the extension to the 60 day limit is denied, the money will be treated as a contribution. And if it is more than $7000 (or $8000 if over age 50), or is more than your compensation from working, it will be considered an excess contribution that should be reported on form 5329. And as long as you have not removed the excess contribution, there is an ongoing requirement to file form 5329 and pay a penalty every year that the excess remains in the IRA. So the statute of limitations essentially does not run out until 3 years after the IRA is spent out and closed, or 3 years after the excess contribution is removed. While it is true that most people are never audited, it can still get really bad if you are one of the unlucky few. So if you want to self-certify an extension to the 60 day window, make sure you can support your reasons.
