Advance payment without reference to a sales order

Dear gurus,

we have a customer who does several orders each month, but we only create only one invoice at the end of the month for all orders. This works fine.

For various reasons we would like to have that customer pay us a fixed amount in advance on each 15^th of a month, let’s say 50.000$.

Now, we would like to subtract those 50.000$ in our monthly invoice, e.g.:

INVOICE

Item 1, order 1          10.000$

Item 2, order 1          20.000$

Item 1, order 2          15.000$

Item 1, order 3          30.000$

--------------------------------------------

Sum                        75.000$

Advance payment    -50.000$

--------------------------------------------

Amount to pay         25.000$

Is this possible in SD?

Cheers

Alicia