The manager isn't owed the option of declining OP's plans. The manager can decline to give leave, but if it's simply to prevent OP from attending something tha

The manager isn't owed the option of declining OP's plans. The manager can decline to give leave, but if it's simply to prevent OP from attending something that the manager disapproves of, then it's sheer inappropriate cattiness. Unless OP is at risk of exposing privileged information about their company, the manager has no business trying to interfere with OP.