Remember these codes:-
<rep> : repeating command block
<chain>: chain command block
<imp>: impulse command block
<con>: conditional command block
<Xcon>: non ""

Try this:
<rep><always active> /testfor @p[r=100] {SelectedItem:{id:"minecraft:string [or any item]"}}
<chain><con><always active> /clear @p minecraft:string 1
<chain><con><always active>/give @p minecraft:bow 1
--------------------------------

What does this do?

Ans :- This exchanges the item with a particular reward you want. In this way, you can easily get the item by selecting it!

thank you,
-pyrotrooper14