Retention Bonus to monthly after getting revised compensation letters?

Isn't this ridiculous. Can a company take our money? It was included in our CTC and stated will be paid after we complete an year. If they make it monthly effectively from July 1st, then about a component that they were supposed to pay after the whole previous year? Can we do something about this? Mail HR or someone if they dont pay us in July. Ideally if we they want to make it effective, they should be doing it from August and not July, or clear as arrears.