Getting correct information from other DB

I created a "Faktuur" system. (Faktuur=List of work done by customer, and the cost). This is related to a DB with Name and Adress data (Contact DB) of all our customers. The faktuur gets the name etc from the contact DB, which works good.

Now i created a payment reminder. It's a sheet in the contact DB wich is letter form in which several items are editeble, it auto adds current date in the letter and name. I created a relationship that allows me to have an pulldown menu in which you can select the fakturen which the client has with us, and whether or not he payed them. I used a Debet Nr, which is auto-assigned in the contact DB and used in the factuur to identify the customer. So this pull-down menu shows all fakturen with thesame debet nr, and thus thesame customer.

Next i created a short summary of the faktuur, with the subtotal taken from the faktuur, than createde a calculation to add the adminstrative cost (usually 15,-) and BTW and add them all up to have the amount that the customer needs to pay. My problem is that for this calculation it needs to take the amount of the factuur previously selected with the Factuur Number, but it takes the first factuur it finds, instead of the previously selected one.

First i thought i add a relationship with Factuur Nr, then the calculation works but it can no longer see other fakturen with thesame debet nr.

I hope i explained the in a way people here can understand, i will be happy to clarify anything that is unclear.