How do I... countunique (qty of customers)...

Hello!

Important report for us:
I do want to know:
how many customers each salesman had in a period e.g. 1.4.12..1.4.13, which bought (from him) for total amount more than 500 EUR?

I tried this:

=NL("CountUnique";"Debitorenposten";"Debitorennr.";"Verkäufercode";$C8;"date";$E$3;;;"Link=";"Debitor";"Nr.";"=Debitorennr.";"Verkauf (MW)";$E$5;"Verkäufercode";$C8;"date";$E$3)

I linked to "Debitor", cause I need to know, if that customer bought for more than 500 EUR. But unfortunately then I do not know, if that customer bought those total amount at this one
salesman (or maybe from other salesmen too).
So: normaly I could get all information from "Debitorenposten", but I want only the customers, who bought in "sum" min. 500 EUR from that one salesman, e.g. Mr. Miller.
Any suggestion :?
Thank you very much and kind regards :)
Klaus