If you are having difficulty with TSP on this matter, I believe you also have the option of withdrawing the excess from the other employer plan that you participated in....they may be easier to deal with:
Participation in plans of unrelated employers. If an eligible participant participates in plans of different employers, he or she can treat amounts as catch-up contributions regardless of whether the individual plans permit those contributions. In this case, it is up to the participant to monitor his or her deferrals to make sure that they do not exceed the applicable limits.
Example: If Joe Saver, who’s over 50, has only one employer and participates in that employer’s 401(k) plan, the plan would have to permit catch-up contributions before he could defer the maximum of $20,500 for 2007 (the $15,500 regular limit for 2007 plus the $5,000 catch-up limit for 2007). If the plan didn’t permit catch-up contributions, the most Joe could defer would be $15,500. However, if Joe participates in two 401(k) plans, each maintained by an unrelated employer, he can defer a total of $20,500 even if neither plan has catch-up provisions. Of course, Joe couldn’t defer more than $15,500 under either plan and he would be responsible for monitoring his own contributions.
The rules relating to catch-up contributions are complex and a plan participant’s limits may differ according to provisions in the specific plan.
Treatment of excess deferrals. If the total of a plan participant’s elective deferrals is more than the limit, a plan participant can have the difference (called an excess deferral) returned to the participant from any of the plans that permit these distributions. A plan participant must notify the plan by April 15 of the following year of the amount to be paid from the plan. The plan must then pay the participant that amount plus allocable earnings by April 15 of the year following the year in which the excess occurred.
Excess withdrawn by April 15. If a plan participant withdraws the excess deferral for 2005 by April 15, 2006, it is includable in the participant’s gross income for 2005, but not for 2006. However, any income earned on the excess deferral taken out is taxable in the tax year in which it is taken out. The distribution is not subject to the additional 10% tax on early distributions.
Excess not withdrawn by April 15. If a plan participant does not take out the excess deferral by April 15, 2006, the excess, though taxable in 2005, is not included in the participant’s cost basis in figuring the taxable amount of any eventual distributions from the plan. In effect, an excess deferral left in the plan is taxed twice, once when contributed and again when distributed. Also, if the entire deferral is allowed to stay in the plan, the plan may not be a qualified plan.
Reporting corrective distributions on Form 1099-R. The plan must report corrective distributions of excess deferrals (including any earnings) on Form 1099-R, Distributions From Pensions, Annuities, Retirement or Profit-Sharing Plans, IRAs, Insurance Contracts, etc.
Refer to Publication 525, Taxable and Nontaxable Income, for more information about limits on elective deferrals.