"雪中送炭"与“锦上添花” 雪中送炭 means help you in time 锦上添花 means good already， and become better

“雪中送炭”and"锦上添花"both mean： make sb./sth. to be better by giving help or something useful. the difference is that the receiver in "雪中送炭" are very helpless, poor, even can't live without the help. the receiver in "锦上添花" , they have done something good, but they would become better after getting other's help or advice. For example: 小明收到好心人的捐赠，可以支付他爸爸的医药费了，这真是雪中送炭啊。 他本来就擅长医术，又学习了心理学，这真是锦上添花啊。

@mayi Thanks for your helpful answer.

hope my explanation helps