Nothing matters, just as everything. What comes to you comes to you. Stoicism believes In determinism, the choices you’ll make are the ones that you’re supposed to make and there is no right or wrong about it (the physics).

But since stoics do believe in a chain of cause and effect and logic should be used to predict it. But not to your own good, the ideal teaches altruism. You have (conditional) control about the decisions you make, not about the causal chain though.

“When someone throws a roller onto an inclined plane, he does indeed provide the external impetus for movement; but the actual cause of the roller rolling down lies in its shape, that is, in its own essence”

You can’t predict the future. Maybe you’ll regret no matter what you do.

So back to your example with the liver transplant. Stoic ideal would say to give the money away for the good of society. But you also need to accept that you aren’t the stoic ideal, you are selfish and maybe will regret your decisions. So before thinking about “what would a stoic do in this situation” the focus should be on becoming a stoic? I’m not an expert tho so take what I said with a grain of salt.