Help Hooch walk properly again

We got our beloved dog as a puppy just over 3 years' ago from the animal shelter The Last Lampost. They had the most applications ever for this beautiful blue-eyed 'French Mastiff cross' who loves to sing and swim and is the most affectionate dog we have ever owned. He has so much personality and brings us endless joy. But he damaged his back leg jumping and needs cruciate ligament surgery. At first we thought it would be about NZ$2.5K but if we don't want it to happen again and given that he is so young, we need to have a more permanent solution which is either $4.5K or $5.5K - way more expensive than we anticipated. Any assistance would be very gratefully received and we will find the money to top up the difference. Unfortunately I am only working part0time and my husband's job has been decimated by the absence of arts funding for the events industry. It breaks our heart to see our dog in pain and limping, staring wistfully out the window. Please help.