Is it permissible in Islam to put animals down for reasons such as cost or pain?

Assalamu alaikum, I hope you are well inshallah. I have a question I was hoping you could help me find the answer to inshallah. A friend of mine is studying to be a vet and on completion of their studies they will inshallah work at a vetinary hospital for animals. One of the concerns they have is that on occasions they will have to put down animals for various reasons. The reasons can vary such as the owner of the animal is unable to afford the cost of the operation, the animal is in great pain and so on. In islam is it permissible to put animals down taking into the above reasons and any others you can think of. Your help in this matter will be much appreciated. May ALLAH reward you for your efforts. wasallam Brother Sarfraz