Kshatriyas were not allowed to take any daan. Karna started his Kshatriya innings by taking an entire Kingdom as daan! https://twitter.com/KarikadaiBoy/status/1247926793274703872
He refuses the offer of the personification of Dharma saying it is his duty to help his friend and then makes a silly excuse and sits out ten days of the war putting his friend in great danger.
He takes a vow he will never refuse to give any daan but refuses to give what his own mother asks him.
He says his life is dedicated to his friend yet when friend is held captive by Gandharvas he runs away from the scene.
The most important duty of a friend is to ensure buddy stays on the path of Dharma. Karna advises his friend to disrobe SIL who is equivalent to mother.