"If that mockingbird don't sing, I'm gonna buy you a diamond ring."
Wouldn't you just kill the mockingbird? Or glue it's beak shut?
Don't tell someone that if their gift doesn't work properly you'll buy them a much better, more expensive present. They'll just sabotage it.
Also, if you like someone enough to buy them a diamond ring if the mockinbird you just got them turns out to be faulty, why not just get them the ring in the first place? If you can afford to offer it, yer just coming across as a cheapskate by telling them that you thought you'd try giving them this crappy, possibly dysfunctional bird first.
Right, I'm off to The Star to for booze and Enders (my mate, not the shit BBC1 soap). Yay!