Where in Harper Lee's To Kill a Mockingbird does it talk about Mayella offering Tom Robinson a nickel?

The idea that Mayella offered Tom a nickel for his services is presented twice in To Kill a Mockingbird.  In her testimony against Tom Robinson (Chapter 18), Mayella states that she offered Tom a nickel to "bust a chiffarobe" and that when he came into the house to do so, he instead chose to attack her.  According to that story, Bob Ewell happened on the scene shortly thereafter.

In Tom's testimony in Chapter 19, he addresses the idea that Mayella offered him a nickel.  He states that she did offer him a nickel the previous spring, but that he chopped up the piece of furniture that she requested without taking the nickel.  As for the night in question (the night of Mayella's alleged rape), Tom stated that he honored Mayella's request to help her fix a door, and that she, in turn, kissed him.  It was then that Bob Ewell arrived on the scene.

Thus, the nickel comes up in both testimonies (Mayella's in Chapter 18 and Tom's in 19), albeit in two different ways.