Denial of annual performance bonus by employer on a medical leave

I was promised a fixed annual bonus by my employer, which I will be subjected to get if : 1. I have completed 3 months of tenure as of 1st April 2. I don't resign before bonus reimbursement period. On 1st week of june I received an email that I will be receiving my bonus amount in two installments one in 2nd week of June and another with June's salary. Since 1st June I am on a medical leave. I have received the first half of bonus on 12 June but they denied to give me the second half of my yearly performance bonus saying that I was not present in the June month. The bonus I get is of my performance throughout the year, the period in which I was present. I want to know about my options here?