Morality and distance

[Letum]

Perhaps a year ago this moral dilemma occurred to me:

If I was in my living room and a guest started to have a violent, natural death and the only way I could save him was by instantly paying £500 from my bank account, I would instantly do so, rather than watch him die in front of me. In fact, I would be willing to risk my life to save some one in peril before me, let alone £500!

However I know for a fact that I could save many lives by giving away £500 to a charity, but I don't.

It would be callas and greedy to watch someone die in front of me because I didn't want to spend money to save them. Is it not equally callas and greedy not to give lifesaving money to charity if I know more people will die if I do not?
After all, the only difference is I don't have to watch people die when I do not make £500 donations to charity.

Last year I bought a motorbike for enough money to buy some anonymous person a lifesaving operation that they could no afford. Does that mean I valued the bike more than the life of another human?
Obviously at the time I didn't know of anyone who needed such a operation, but I'm sure I could have found many in the world if I had looked.

Does anyone know of any writings on the issue?
This is something that troubles me deeply and I would appreciate your thoughts.