passing or typing a value

Hi,
I'm new to Access, I've learned somethings through hurry up trial and error process. Since I'm learning this on the fly, alot of thing I don't know.
My question, is there a way where I can pass a value if a certain condition is met and if not have the user type the value in.

Example: I want the user to be able to input an initial balance if a customer is new. However if the customer is not new, I want the remaining balance of an old purchase to transfer to the initial balance of a new purchase. If that makes any sense.

If you can help me out I will appreciate it greatly.

It is very difficult to answer without a basic table design info. Any how assuming you have a table named Transactions with customerID and balance field you can put a textbox in a form where in customer will enter his customer and in the afterupdate event of textbox you use this command to check whether customer is available already
PreviousBal=nz(dlookup("Balance","Transaction","CustomerID = " & me!txtCustomerID),0)
if PreviousBal = 0 then
'ask the customer to enter new balance in another textbox
else
anothertextbox.value = previousbal
endif