I have written "From May on it will give more money" and it was rejected. I have literally translated the verb "geben" as "to give". Is the expression "... wird es geben" an idiom meaning "there will be"?
Consider the following. This sentence is complicated and, although it could be split into several simpler sentences, if you want to understand general English conversation you will have to deal with understanding sentences like this.
That's not quite the same meaning though. "By May.." implies it could be before May, whereas the meaning here, I think, is from May onwards. "With effect from May..." would be a more formal way of putting it.