Surely as long as your acts are an attempt to increase well being and as long as your beliefs that your acts do indeed increase well being are reasonable then you can be said to be acting morally even if you are misguided.
An evil man has issued you with a box that has 2 buttons on it.
If you press button A he says he will do nothing.
If you press button B he says he will kill 100 people.
If you do neither he will kill 200 people.
You have no reason to doubt him so you press button A.
Indeed he does nothing and sends you a video tape of the 100 people going free.
Behind the scenes you are doing all in your power to capture this man and also to gain further verification that he has indeed set the people free.
Now imagine this again in a parallel universe when in fact the tape he sends is fake. He was lying about the outcomes of your button presses and had you pressed B he would have done nothing but as you pressed A he chose to kill the 100.
Now it seems to me that Sam would say your act in the parallel universe is less moral as it had an outcome which decreased global well being. I would argue that the acts are morally equivalent because in both cases there was a sincere and dilligent attempt to increase well being.
Moral relativism in this sense is surely acceptable. That is to say, if people genuinely believe their acts promote well being and their beliefs aren’t unreasonable (given their culture/brain/upbringing/access to information) then one might say they are moral acts irrespective of their actual success in bringing about an increase in well being.
Did Sam tackle the problem of “intentions” in his book?