I recently resigned from an organization that is requesting I reimburse the tuition reimbursement I have received in the past 2 years.

I signed an agreement letter that basically siad I would pay back the money I received over the past 2 years upon willfull termination of employment. This is a substantial amount of money near 10K.

Am I required to pay back this money. Do I have legal grounds to stand on to decline?

Whther the agreement you signed rises to the level of an enforceable contract is a matter of state law. You probably should have it reviewed by a local attorney, as none of us can comment on something we haven't seen. As a practical matter, most tuition reimbursement agreements I've seen hold up in court. I'd suggest trying to work out a payment schedule with the employer.