Okay, here's my dilemma...

I do bookkeeping for a small business - a women's clothing store.  I took over the job from the 80+ year old husband of the owner.  After a year, I'm still trying to get things in proper order. 

The owner writes a lot of post-dated cheques.  So, for instance, she will place a big order for all the summer clothing with a supplier.  She works out a payment plan with the supplier.  The total order may amount to $15,000.  The payment plan may consist of 10 payments of $1500 weekly for 10 weeks.  All the cheques are written, post-dated, and sent to the supplier.  These are entered into Sage as pre-payments for that vendor.  The clothing arrives in multiple shipments.  An invoice is associated with each shipment and this is posted into Sage. As the sales progress in the store, additional orders may be placed or returns may happen as well, resulting in additional purchases or credit notes - tweaks to the clothing line.  These are all posted into Sage.  Now there is a big mess for some of these main vendors in Sage.  All the invoices, credit notes, and prepayments are piled up for multiple seasons in the vendor account in Sage.  And it's nearly impossible to get it all to match up exactly.  

At the end of each season, the supplier will work out the final amount owed or credited and this is settled with a final payment or a credit that is carried to the next season of orders.

My problem is that there is a year's worth of stacked up invoices, credit notes, and prepayments that don't all match up or zero-out with each other.  Even though, accounts are all settled with the vendor.  I just can't get them to match up and zero-out with each other in Sage. 

What is the best way to clear out these vendor accounts?  And say for instance, as of this date and payment, the account is $0 owed - no outstanding invoices.

Any help would be appreciated.  Thanks!