I'd suggest creating a master category for each grant area. Then enter grant money as an inflow* against a category, rather than as income available this month. When you want to spend grant money, enter a negative figure in the budget for that amount, then budget the same positive amount it into other sub categories as needed. The total budgeted amount for a category should equal 0. If you don't allocated all your money e.g ($1100), you will see the unbudgeted and available to spend amount at the top the YNAB budget screen.
Master category - Environmental Grants. Budgeted. Outflows. Category Balance
Grants Received -1100. 2000*. 900
Field Expenses. 400. -100. 300
Payment for my time. 700. -700. 0
*2000 = grant income