Where is the word play in this dialog?

Solution 1:

Speaker A asks "Can I help you?" and gets the answer from speaker B "Yes, your face, my arse."

This is the joke answer to an entirely different question, namely "Do you have a match?"

Speaker A concludes that Speaker B has misheard "Can I help you?" for "Do you have a match?" so Speaker A clarifies his question by repeating it.

Speaker B admits his mishearing by saying "Oh, I see" and proceeds to respond reasonably to the Speaker A's question offering help.

The joke (such as it is) is that nobody would mistake "Can I help you?" for "Do you have a match?" but both parties proceed to act as if that were a perfectly natural thing to happen.