I'm wondering if your interpretation is incorrect.
The $20k bonus was not a matching or profit sharing contribution... it was a bonus as part of your severance and would have been income to you if it hadn't been put into the 401k (right?)... by crediting it to your 401(k) they effectively let you defer it and caused your contributions to exceed the $18k limit so you overcontributed and they are looking for your cooperation in remedying the overcontribution.
If my interpretation above is correct and their mistake resulted in you unknowingly overcontribute for 2017 then it might make fixing it easier... you would withdraw the overcontribution and pay the resulting tax and 6% overcontribution penalties, etc. and it would be a 2020 event. They should make you totally whole including any necessary grossups for taxes, all penalties, tax preparer and legal costs, etc. since it was their mistake.
I'm thinking that the treatment would be akin to an overcontribution to a deductible IRA since you were not taxed on the income.
https://www08.wellsfargomedia.com/a...planning/correct-excess-IRA-contributions.pdf
I believe the $20K was a qualified non-elective contribution. I pulled the relevant section in the plan that covers this:
5. Discretionary Nonelective Contributions
a. Ratio of Compensation Formula
Nonelective contributions, if any, made to the Plan by your Employer shall be allocated to your Account based upon the ratio that your Compensation bears to the Compensation of all eligible employees within the group of eligible employees to which you belong. Each employee will be considered his or her own group.
For additional information regarding the computation of this benefit, please contact the Plan Administrator.
6. Other Contributions and Limitations
a. Qualified Nonelective Contributions
Your Employer may designate all or a portion of any nonelective contributions for a Plan Year as “qualified nonelective contributions” and allocate them to certain Non-Highly Compensated Employees to help the Plan pass one or more annually required Internal Revenue Code non-discrimination test(s). You will be 100% vested in these contributions and may not request a hardship withdrawal of these contributions.
b. Additional Nonelective Contributions
Your Employer may be required to make a flat percentage Nonelective contribution to you if you are not a Highly Compensated Employee due to non-discrimination testing.
c. Limit on Contributions
Federal law requires that amounts contributed by you and on your behalf by your Employer for a given limitation year generally may not exceed the lesser of:
$54,000 (or such amount as may be prescribed by the Secretary of the Treasury); or 100.00% of your annual compensation.
Your Employer may make discretionary nonelective contributions in an amount to be determined by the Board of Directors for each Plan Year. You must complete at least 1,000 hours of service during the Plan Year and be employed as of the last day of the Plan Year to be eligible to receive any nonelective contributions that may be made for that Plan Year. You do not need to satisfy this requirement if you die (including death while performing Qualified Military Service),
become disabled or retire during the Plan Year.