The donors give money to specific candidates for specific reasons. The money is theirs, not the candidate's. A candidate cannot "promise" to give this money to another candidate any more than they can promise to buy themselves a one way ticket to Buenos Aires with that money. Whether it's legal or not, it is unethical to break the bond between donors and a candidate without as much as having permission to do so.

A Political Committee, including a Presidential campaign, is limited to a $2,000 contribution to another candidate. The balance can go to the National Party.

was in the Senate?

...which can be used for any Federal campaign of that candidate. That's why Sanders/Klobuchar/Warren could transfer funds from their Senate races to their Presidential race.

I always thought this is what the Orange Meance was/is doing. He's amassing a war chest to spend on himself and once he looses keeps it and runs it like he did his Foundation.

I'm sure he's laughing all the way to the bank to have all his supporters give him free $. Hell he couldn't even do this selling condos.