We can't really answer number 2 as we don't know what you've set for your calculations in Excel and don't know just how much of a "Newbie" you are.
Using a combination of summary and calculation fields, it's possible to set up a ledger so that each approved PO is a ledger entry in the appropriate fund. It's even possibile to set this up so that a single PO draws from more than one fund.
Using just the calculation you describe, you can set up two calculation fields and a summary field:
cFinalCost: Discount cost + ShippingAndHandling
cBal: FundsAllocated - cFinalCost
sRunningTotal: Summary field, running total fo cBal, Restart summary for each sorted group. Group by FundID.
FundsAllocated is the field where you record the initial funds and any additional amounts allocated as time passes. Each expense and each fund allocation is a separate entry in your table just like deposits and withdrawels in a check register. You sort and group your records by Fund if you want to see the balances of more than one fund at the same time.