Your budget is an estimate of categories of expenditures, with associated amounts. During a project, budget amounts will probably change somewhat according to actual expenditure needs.
For example, unspent salary and benefit dollars may shift to the current expense category; a piece of equipment might be deemed unnecessary and the dollars may shift to current expense; or the scope of the project might dictate a different change.
Most of the time, re-budgeting requires permission from the granting agency, which will be requested by OSP. Again, it is very difficult to increase the salary dollars, so we encourage you to avoid that change.
A paper trail is required. This is accomplished by notifying your Sponsored Project Administrator in writing (email can suffice) so an auditor will be able to verify that the decision to change the budget was made by the PI.