It may sound silly, but a big deal 4 me...

[jav88]

Hey there....I started using AGS a week ago and fortunately I understood all the topics in the tutorial and managed to use the program....but there's something that seems to be impossible for me....
  I want to give something to a character in order to make him give me another object, so...how do I do that??? Please help me. Thanks bye,
JAVIER.

[Scummbuddy]

LoseInventory?
and try and make subject titles that have to do with your problem

[Scorpiorus]

I want to give something to a character in order to make him give me another object, so...how do I do that???

Under the character use inventory on character interaction:

int active_item = character[GetPlayerCharacter()].activeinv;

if (active_item == _SOME_INVENTORY_YOU_HAVE_TO_HAVE) {
SetActiveInventory(-1);
LoseInventory(active_item);
AddInventory(_NEW_ITEM);

}

~Cheers