Qitzur Shulchan Arukh – 183:2
Even someone who incurred damages on himself he can not remove it from himself if through this he will cause [damages] to come upon someone else. Because he can’t save himself even through indirect causing his friend monetary damages. However, before he incurs the damages, he may push it off that it won’t come upon him, even if through this it will be incurred by his friend. For example, if a stream of water approached [threatening] to flood his field, while it hadn’t yet entered his field, it it permitted to place a damn before it even though through this it will flood his friend’s field. However, once it entered his field, it is prohibited to remove it in a way that it would enter someone else’s. Since the damage already occured, it is inappropriate to divert it from him and place it on his friend.
All else being equal, one may save their own first. However, if I already incurred damages, then all else isn’t equal. I’m merely using my misery as an excuse to share it with someone else.