Find A Relation Between 2 Diff Tables

hi
i have a database to manage utility bill payment , it consist of
- Bills :
- billID
- Benificiary Name
-Cost Center
- Bills Transaction :
-TransID
-bILLid
-BillDate
-BillAmount
-Payment Transactions :
-PayTranID
-bILLid
-PayAmount
-PayDate
-BankRef

i made a union query from Bills Transaction and Payment Transactions to calculate bills balances which is : billid,sum(Bills Transaction.BillAmount)-sum(Payment Transactions.PayAmount)
all is working well , but the problem is i cannot find any relation between billtransaction
and billpayment ( per bill ) , cause i wish to payment details for each single bill transaction
the normal case is : bills issued as monthly basis but may fully or partially pay as the following cases :
- each bill transaction may fully pay one time
- in some cases : each bill transaction may fully pay but in multi settle
- multi bill transaction (per BILLID) may fully pay one time
IN CONCLUSION : each bill transaction should be stteled fully within one or two or maximum 3 months , say bill balance for each bILLID shall be zero.
how i could find a relation between this two transaction ( bills and payment ) to preview
payment information for each single bill transaction
exapmle :
billID : 39
BILL Transaction BillPayment BillsBalance
Bill Date - Amount PayDate- Amount
jan08 - 1000 1-1-2008 1000 0
feb08 -1200 5-2-2008 800 400
15-2-2008 400 0
mar08 1900 1900
apr08 1100 30-04-2008 3000 0
may08 1200 05-05-2008 900 300
jun08 1300 30-06-2008 1600 0