It is possible, but it is voluntary on their part: legally, under the typical tuition reimbursement agreement, you owe all the money right away; the employer can choose to give you time to  pay, but does not need to do so. All you can is negotiate with them and try to come to a mutually acceptable plan. If you do, get its terms in writing and make sure you 100% honor it (if you miss a payment, the rest will come due immediately).