"It is merely a difference in amount, not in kind."
The application of any rule must be tempered with reason, and reason tells us that there is a great difference between $500 and $50,000. We must look at the difference between pushing a little old lady into the path of a speeding bus and pushing a little old lady out of the path of a speeding bus, instead of treating both parties as "lady-pushing criminals."
Further, since we are considering a question of morality and not proposing legislation, it's not even necessary to decide where the lines lie. Is $5,000 too much? Or is it $10,000? Since we're not telling people what they must do, we are at liberty to apply our reason.
"you've also expressed your belief that a person has no obligation to tend to the needs of others."
Rather, I have expressed that I do not believe we should have a legal obligation to help others. I do believe that it pleases God when we help others voluntarily, just for His sake, and it displeases Him when we could help but refrain from doing so out of selfishness.
"But what about spending that does not generate any capital? A party, or a lavish gown for his wife, or a worldwide cruise?"
Once again, I am not advocating any legal coercion. At the same time, it saddens me to think of the good that could be done with that money, were the rich person so inclined. Of course, most rich people do some good whether they want to or not, just by keeping the economy ticking along.
"I guess you missed the part I posted about capital formation, and how it benefits all, even the starving third world child."
No, I just accept it as axiomatic, and saw no need to comment.
"...the surgery might advance medical knowledge for humans, the $50,000 goes to the coffers of prosthesis maker...gets invested...fund new materials, or research, which benefits mankind with new human prosthesis, or new drugs, or more efficient farming techniques."
All true. It is an ill wind that blows no good. However, the good things you mention are, IMO, attenuated, in comparison to a direct contribution for the benefit of mankind, which could actually save a life now.
"What I do think is immoral is telling other people how to spend their money."
I think it is immoral to pass laws requiring that people spend their money in a certain way.
We are so PC these days that a person can be brought up Catholic and never even hear that for close to two millennia "rebuking sinners" was one of the seven spiritual acts of mercy.
In other words, it is immoral to use force, but not persuasion.
"We all do it from time to time, myself included, but we're all better off when we let capitalism take its course."
We're better off with the maximum level of freedom, but to use that freedom responsibly we must develop a properly informed conscience. This requires study and the interplay of ideas with our fellows. That, in turn, will eventually require us to tell someone "What you're doing is wrong."