I recently had offer to gift 600 credits to a friend, which I did, and the credits went from my account. Unfortunately he never received them and I have no idea how to resolve this
Goose, there is NO option to gift credits to your opponent after a match. What you are seeing is the option to gift a sleeve of balls to them and the credits it will cost you to do so. You gifted your friend a sleeve of balls. They removed 600 credits from your account to pay for them and he should have received a sleeve of balls. He may not have noticed as the balls you are given the option to gift are the ones your friend currently uses so he might not have noticed his inventory increase by 3 balls.