Can I be denied a benefit offered by my employer due to my disability

My employer offers tuition assistance, available to all employees who have been with the company for 6 months, the employee is in active status and is in good employment standing which I do qualify for. In order to receive reimbursement for tuition we need our manager to sign the request. My manager refused my request due to my disability, stating it could interfere with work. My disability doesn't interfere with my responsibilities at work or my job performance. I have been with the company for a year and half and all my reviews have been excellent. I even offered to have my physician certify that taking two online classes would not cause my disability to worsen and lead to poor work performance. Can I legally be denied this benefit due to my disability?