nadya,
what you are looking for is a substantial system. we developed ours from the ground up with issues like this in mind. we base out documents, purchase orders-purchase invoices-receipts (GRN's), on a stage basis with links between each so that we can follow them and report on them. simply put we look at GRN's that do not have a purchase invoice linked to them and use that. to be honest without knowing your system i could not recommend anything specific, its the end of day here so i will be offline until tomorrow but i am sure that there is someone here that will be better able to help, maybe mentioning this in the chatter section in foxpro may get more people to look at it. i am curious, what financial package are you using, or are you developing a new one?
Cheers
~M