A needle aspiration of the bladder is quite common to obtain a sterile sample of urine. A catheter can also be used to obtain a sterile sample. I'm surprised the vet said a catheter would cause an infection. In my experience, and unfortunately I've had a lot of experience with doggy urine infections, a catheter won't cause an infection although it can cause irritation. Collecting a sample of urine when a dog wees can show a false positive for infection because of what can be found on the skin and isn't usually used for lab testing. A urine infection will usually make a dog wee a lot more and also drink a lot more which are the main symptoms to be aware of.
@Anne123 I hope the morphine injection kicks in quickly Anne and Finn starts to improve. It's just awful to see them in pain and not be able to help. Everything crossed here.