hi,

see it is really a tough decisions you have to make. I assume you are in post graduation , so it would take at least of two years to complete. now gramin dak sevak job is pure government. so now you have to chose between the job and the your desire to complete the higher education. mba in agribusiness management is quite demanding and have excellent career  prospect available in agricultural industry. so if you have confidence that you can make a better career in that field I would suggest to continue the studies. unless you may leave to join your job.