You need to change the service hours on the budget to match it with the contract. This can be done from the Projects > Budget screen. These are calculated as:
(Hours x Bill Rate) x [1 + (Tax 1 + Tax 2 + Tax 3)]
After you change the service hours on your budget, the budget amount gets changed accordingly. You can't directly edit the contract amount; you have to modify the service hours or bill rates to match the budget amount to the contract amount.