The box shown below cannot be changed within the estimate. Here's why....
Why can't I change the estimate values in a job
In our mind, once you accept an estimate and start managing the job there is no need for you to adjust the budget of that estimate unless you have variations. To expand on that, once you accept an estimate, you're then moving onto tracking just how much you spend, which in Buildxact is actual cost. Especially as people learn Buildxact and the best ways to navigate it there was a risk that users would add to the budget of a job without understanding the consequences, meaning other numbers/percentages in the app could be off.
What were the risks of changing the estimate value
It all comes down the calculations Buildxact does which indicate to you how much budget you have left to complete the job, and also the percentage which indicates how much of the budget you have so far spent.
The short version is that if you inflate your budget then Buildxact would tell you that you both have more money left to spend than you do, and that you've spent a lower percent of your budget than you have.
If we go into more detail, we need to look at the different items involved in the calculations. If you've never seen the below screen before, you can open it by clicking the arrow (in the red box in the image below) down the bottom of the actual costings screen within any job.
The total budget for the job, made up of the original estimate budget plus variations plus any manual additions. Maths here is  +  +  = 
The original estimate budget
The budget added from any accepted variations
Any manually added estimate values, it's this item we have now stopped.
The total actual spend on the job, made up of part-received/received purchased orders plus completed work orders plus any many additions. Maths here is  +  +  = 
Total purchase order spend made up of part-received/received orders
Total work orders spend made up of completed work orders
Manually added actual costs
The amount left to spend before the budget is exhausted. The maths here is  -  = 
The percent of the budget which has been spent. The maths here is (/)x100=
So, after lableing all the different elements, the problems that could potentially occur were:
That  would increase, hence making  larger, which in turn makes  larger and tells you that you have more money left to spend than you do.
Also  would be affected by  getting larger so the percent of the budget spent would be shown to be lower.