Handling Transfers

I am attempting to create a database that will mainly handle transfers of
materials between dealers. This is an aerial application business, so
dealers bring us chemicals that we apply on crops. Dealers are responsible
for providing us with the correct amount of chemicals for the fields they
need sprayed. We can't always use their specific chemical. All of our 15 or
so dealers bring the same chemicals, so we often use one dealer's chemical on
another's field. I need to keep track of this debit/credit system, so I
always know who owes who what. I am having trouble implementing this idea
into Access so any transaction or transfer will deduct and add the chemical
to the dealers involved in the transfer.
Basically like this:
Dealer A transfers 40 gallons of chemical Z to Dealer B. Both Dealers had
60 gallons of chemical Z sprayed for the day.
At the beginning of the day Dealer A had 120 gallons, Dealer B had 50.
So, after all is said and done, Dealer A should have 20 gallons, Dealer B
should have 30.
I need a way to keep track of the transfer and the amount sprayed for each
day.
Any thoughts and suggestions would be much appreciated.