BigBallinStalin wrote:In other words, people should be morally obligated to donate some amount of their goods to very poor people since this increases the net utility of the world.
Let's address the situation where donating can conflict with the means of creating wealth for the very poor.
To be clear, a donation is a transfer of wealth. In terms of wealth alone, this is zero-sum. In terms of utility, this is positive-sum (given that the donation is voluntary, of course). An exchange is a creation of wealth because in an exchange ex-ante each party values the other party's good more so than their own good. After the exchange, each party has increased their value (i.e. wealth). In terms of either wealth or utility, they are both positive-sum. We're assuming that utility is an increasing function in wealth and donations.
Now, suppose you pay $10 for a T-shirt that's been imported from an Indonesian 'sweat-shop'. Your $10 becomes distributed among the chain of suppliers--from the retailer, shipper, sweat-shopper, and the suppliers of labor within each market. Consequently, your exchange increases the net wealth of the world, and net utility has increased since additional wealth causes utility to increase. Net wealth: >$10--in terms of total value. Net utility: +something + wealth (i.e. the value created from exchange).
If you were instead to donate $10 to one of the many workers in the sweat-shop, then you forego the opportunity to increase the wealth and utility of all the other very poor workers in that sweat-shop as well as everyone (of varying wealth) in the production process. In other words, donating has a cost (the opportunity cost), and an exchange has an opportunity cost (from whatever else you could've done with $10). In this example, net wealth: $0. Net utility: +something + 0(wealth).
In short, since we live in a world of scarcity every action of ours incurs a cost. In the trade-off between donating and trading in terms of maximizing utility, trading increases utility on average more so than donation. So, why not opt for 100% trade and 0% donation*?*tangent: and for that matter, why not advocate for the abolishment of international trade restrictions?
Your conclusion simply does not follow from your premises, for several reasons. First, you incorrectly assume that utility creation is not counted positively as part of an exchange, but this is wrong to the extent that it might make me personally happy to donate money to others. Therefore, the donation does create wealth, if we broadly count my happiness as part of the world's net wealth. (And there are practical reasons to do so -- my happiness might translate into increased productivity, say.) Second, while in the exchange I do have the T-shirt, this is offset by the costs of producing the shirt; the $10 is therefore paying the cost of labor, but if I had just donated the $10, that time could have been spent producing a T-shirt for someone else, so they get both the $10 from my donation and the $10 from somebody else buying a shirt. This assumes that the T-shirt market is not affected by my individual decision not to buy one*, which seems obvious given the number of people buying T-shirts in the U.S. Third, your argument about how only one person gets the money is a non-sequitur; if these people are of similar wealth then net utility is increased the same if I give it all to one person, or split it equally among the workers. But in either case I could just split up the money among those workers the same way, and receive the same net effect (minus the T-shirt). Fourth, in any likely scenario, a good fraction of that $10 will not make it to the poorest workers, but will stay with the managers, who don't need it as much. Fifth, there's not even an opportunity cost. We're not talking about whether someone is going to buy a bunch of T-shirts that they don't need, as an inefficient method of cash transfer to the poor, are we?
*Many of my arguments assume that we are talking about this on the typical individual level. If you are an individual with enough leverage over the market to actually change the T-shirt market, then we need to think about different ways for you to donate.