I would like to get extra water because I tend to sweat a lot and this makes me dehydrated. 
Okay, I can agree to that.  I'm camped out next to a stream so extra water is less important for me.
What is your high priority item? I am willing to give extra firewood. 
I prefer extra food because I will have some friends joining me in a few days.  Firewood is also somewhat important for me though.
How many friends are you expecting? I have a pretty large family with me. 
2-5, I'm not entirely sure yet.
I have my ma (she's getting up there in age), my husband and our two kids. Then I have my two little sisters. And me of course. I would like to have a little extra food to feed them all.  
Okay, I understand.  We all want to be safe and not run out of food.  How much do you need firewood?
Firewood isn't so much a priority. I'm willing to give up some of it if you're in need.
Alright, hmm.  Can I have all the firewood and 2 food items?  Then you can have all the water and the 1 extra food?