Now think about this whole situation. Why was my friend disappointed and angry? After all he didn't have any use for the guitar himself, did he? He wanted to make the musician happy and he did, so where is the problem?
The mana. The spiritual force that imbues gifts and represents the gifter in it.
Your friend didn't simply want to offer money to his friend, he wanted to share a thing (an object, a function, an activity), and to have a bit of himself over there. His friend redefined it by purging the object of its symbolic value, and of its bonding/sharing function. He made it a neutral, monetarizable commodity. It's common "social" sense to not sell (or further give away) objects that are imbued with mana, that is : you don't give gifts further away, or you don't sell away gifts, because it severs the symbolic tie attached to the object, and constitutive to the gift action.
It's colliding two realms. On one hand : the realm of gifting (the reason why, in general, you don't offer cash to your friends at christmas), which is about the social bond sustained by reciprocity and perpetually asymetrical reversible debts (you keep 'owing' your friends who keep 'owing' you). On the other hand : the realm of commerce, where the objects are being exchanged through carefully quantified equivalence of value, to prevent any 'debt' to last, and to allow for stopped relationships (you don't feel like owing anything to a merchand/client after a transaction, they can exit your life). This is why, for instance, "paying" a gift through the exact sum of its value or through the exact same gift in return is being percieved as an act of separation. And that's why you remove the price labels on the stuff you offer.
That friend took a gift, and transformed it into a commercial commodity. He wanted an impersonal profit, your friend intended a social act. Your friend saw the "guitar" as a token, his friend saw it as a "type". There was a disagreement about the meaning of the gift, and about the nature of its object.
In short : your friend gave a piece of himself, which was treated as an impersonal item.