There is a big difference between the two. When you say “grant me leave for Friday”, what you mean is that you would like to have Friday off. In other words, you would like your boss to tell you that you needn’t come to work on Friday. The second sentence, “grant me leave on Friday” is rather ambiguous. It could mean that you want your boss to inform you on Friday that you have been granted leave. You don’t want him to tell you today itself as to whether you have been given leave or not. You want him to give you the good news on Friday.