Am I legally bound to repay my company for their tuition reimbursement?

My company offers different tuition reimbursement plans. I am under the plan that the company pays 100% of the tuition and textbooks UPFRONT (before completion of courses).

I was recently reimbursed by the company for my summer courses (which is still ongoing) but I decided to leave company.

Company is asking me to repay them for the reimbursed tuition and textbook costs. They say I am obligated to reimburse them since the courses will run past my last day of employment with company.

I didn't sign any contract. There were no clauses in the employee handbook that states employee must stay with company for X period or will have to reimburse company.

I was under the impression that any tuition reimbursements are not subject to repayment.

The language HR cited from the handbook regarding their position to seek repayment are as follows:

1. Expense reimbursement is contingent upon continued employment

2. If you voluntarily terminate your employment, you will not be eligible for reimbursement

At the time of the reimbursement - I was still employed and have not submitted my resignation.

Am I legally bound to repay company back for their tuition and textbook reimbursement? The language they cited appears vague.

Can I fight to win this? Please help.