posting of invoice

Hi to all

I have an querry regarding invoice verification.

I created po for 100 quantity and did the gr for 50 items . but we did the invoice verification for 100 items. when we post the invoice it gave the message as invoice is blocked for payment. upto this it is correct . when we go the disply of that invoice in header data payment tab the payment block shows as "Free for payment" why it happens in disply of invoce

Regads

Raju