you may need to consult with a lawyer on this one --you definitely have to consult your spd (the plan documents for the erisa plan, also known as summary plan description, hence the acronym). There are competing legal considerations because the plan has the right to a use-it-or-lose-it provision but also the law despises a forfeiture and if you don't get your money back then there is an argument of unjust enrichment for the plan. do not sit on this, speak to an erisa lawyer quickly.
I definitely think that an Employment Benefits specialist would be best positioned to help you. However, what you are really asking is a tax question, and a tax attorney like myself could assist you with the matter. There is a Summary Plan Description which sets out all of your rights under the Dependent Care program. You appear to have already found the section on Qualified Expense reimbursement or called HR and been informed of the 90-day limit. Not to worry, you are not out of luck yet.
You may be able to deduct these Dependent Care expenses outside of the Dependent Care plan. These should qualify for the Child and Dependent Care credit. You can elect to take this credit on Form 2441.
You will note, as stated in Publication 503, that there is a cap on the credit of $3,000 if filing single and $6,000 if filing joint.
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