it wouldn't be logical unless the owner set the budget. you are the GM and if you make a change in thought midsream, i don't think the game version of you should prevent it. may need comissioner mode on if my common sense isn't applied in game, lol.
key with budgets that can only be set in offseason... get things done earlier in regard to FA. you shouldn't be up against the last day to make decisions regarding these budgets.
the last day you can make any chocies, do review them.
i don't have total recall of the exact details of how this works. ther are ways to deduce what is happening by what the game tells you... if it says you only have 5m to spend and you have extra, you know it's forcing you to adhere to that budget. powers of deduction will allow you to figure this out with the feedback given on your screen as you read it. it may be faster to make a restore point, zoom out, look, then restore... answered and can make a better decision with more knowledge.
you can just avoid the situation altogether too by getting things done earlier in regard to payroll expenses. then it's whether or not you overpay at the trade deadline like the rest of the peanut gallery
if you do, you'll want ot earmark some space, otherwise you can just maximize budget for draft picks and scouting etc without fear at end of offseason, because all financial obligations are hammered out before then.