Each grant has an approved budget from the funding agency. The funds are to be spent according to that budget.
The extent to which a budget may be modified once an award has been accepted varies from agency to agency. For example, some contracts from the State of Michigan do not allow for re-budgeting without prior approval; whereas some federal research grants allow for a great deal of budget flexibility. Keep in mind, however, that significant budget revision, while strictly speaking might be allowable under the terms and conditions of an award, may in practice also represent a change in the scope of objectives of the project that requires prior sponsor approval. For that reason, always contact a Grant and Foundation Relations staff member prior to deviating from the budget.