Purchase invoice with negative amount

Hi EveryOne, How we can purchase an item with negative amount ( the vendor/Customer  shoul pay us to take from him an item (recycling items) after that we will sell this item. we should pay the transpot charges for this item. the transport is the only cost we have. i'm asking if there is a tips to do that in nav? , i'have tried to create a sales invoice with negative qte + item charge (pickup item) as amount the vendor /customer should pay us. but i'm unable to create a purchase invoice for transport (the cost to apply for these item)