BT BB 84a(7) :אי לאו דאוניתן לא מצית הדרת בך השתא דאוניתן מצית הדרת בך‏ : had you not acted fraudulently towards me, you could not have retracted; now that you have acted fraudulently towards me, can you retract?
See the entire entry for yny vb. C to vex