Currency Exchange exercise doesn't test edge case in instruction 3

Instruction 3 in the Currency Exchange states:

This exchanging booth only deals in cash of certain increments. The total you receive must be divisible by the value of one “bill” or unit, which can leave behind a fraction or remainder. Your function should return only the total value of the bills (excluding fractional amounts) the booth would give back. Unfortunately, the booth gets to keep the remainder/change as an added bonus.

The edge case for excluding fractional amounts is not tested. Current tests are only for simple multiplication.