According to the Cambridge dictionary send back means: "to return something to the place it came from" and according to the same dictionary "feed back means: "to put something back where it came from"
“send” emphasizes the action of making something to go from one place to another place.
“feed” emphasizes the purpose of helping something grow.
Hence, “send back” emphasizes to return something; “feed back” emphasizes to return and help something grow.
I had to send the shirt back because it didn't fit me.
As profits are fed back into the trust, it sells shares to workers and reduces its own stake.