HUGE decision - keep trying for the child I so desperately want or stop trying and potentially save a life

A few years ago I had a cheek swab done and entered this database to possibly be a bone marrow donor. We were told it was VERY unlikely that we would ever get a call as a match. Well.. this morning I got the call. I'm a potential match. There's apparently like an 85% chance everything will work out and I can save this man's life. The problem: it's about a 6 month or longer process and they would ask that if I agree and they start the process, that I try not to become pregnant during that time, as then I wouldn't be able to donate and the man would be left preparing for a transplant with no donor. My husband and I have been trying for over a year and are just starting fertility treatments. I dont know what to do. I would LOVE to donate and help save a life, but the idea of taking a break from ttc is heartbreaking. I've already waited so long. But then what if I say no and 6 months pass and we still aren't pregnant anyway? Then I turned down a chance to save a life for nothing. UGH. HELP. 😭