I just went to collect an item I bought over Ebay and wanted to tell the seller that I had the exact amount of cash with me and he didn't need to worry about handing over change. How do I say this? I'm sure I heard a phrase once, but it's escaped me now.
Where I live, you would say
Ich hab's genau
Ich kann es Ihnen genau geben
In context when you are shopping in a store, often prefaced by "Warten Sie" (to stop the cashier from searching out the change to give you - actually, this would have applied to your situation as well)
"Genau" is of course the German word for "exact(ly)"
A somewhat more formal term is abgezählt:
Bitte halten Sie das Geld abgezählt bereit.
meaning that one is supposed to hold the exact amount ready in order to be able to pay quickly (e.g. when many people are waiting in a line).
However, saying "Ich habe das Geld abgezählt" is somewhat unsual (although not incorrect).