To avoid a finger injury in beach volleyball, there’s a couple things you can do.
The first place where most players get injured is on the block. If you press your hands over the block and keep your fingers extended and solid, you won't get injured. If you go fingers first to the block, then it's going to hit you here and cause an injury. If you go palms up, it can potentially cause a thumb injury. So the best way to avoid a finger injury on the block is to go over the net, hands fully extended. Anything with weak fingers equals injury.
Pokey is another place where you can get your fingers injured. A lot of players will kind of half form a poke, and that's when the ball will hit and cause an injury in either the knuckle or the fingers. Make sure your hand is fully formed in a full poke to get good contact on the ball on the tops of the fingers and the thumb to avoid injury.
The cobra is another way you can do a pokey, but if this isn't fully formed and the fingers are like this, that will cause injury. You need to make sure that you have full extension here, solid, to contact the ball on this knuckle platform to avoid finger injury. On the hand set, if you do decide to hand-set, the potential for finger injury as well. You're going to have your fingers loose on contact and finish all the way through the ball. If you let the ball hit you on the tops of your fingertips, that will cause an injury.
General advice, if you have weak fingers, is to do finger strengthening exercises like squeezing a ball, physical therapy kind of style. You can also tape your fingers if you have a pre-existing injury, to help strengthen it. You can tape them to one another, too. Some players may get injuries with hand-setting. Tape the pads of their fingers because another way you can get another injury on your fingers is cracking your fingertips when the conditions are cold, whether you're hitting a lot of balls. So you can prevent cracking by taping the finger pads and moisturizing. You can also prevent cracking by wearing a glove. I had to do that for a little while, actually wear a baseball glove on one hand because it cracked.
It's not the prettiest, but prevention is the key. If you can't prevent, then treating it right away before it gets bad.