At the start of ML you need to set "management" (don't know the item name in English version) to ON (default is OFF). Otherwise you won't be managing the money and the chairman/owners define your budget...which sucks ^^ It's the same like when you play Be A Club-Boss, where you set the budget each year...that's been done to you!
Unfortunately you can't change that setting now :/ Maybe it's possible at the beginning or end of a season...don't know. But I guess you'll have to restart if you want to enable this.