When I used to charge per song instead of by the hour, I would never give anyone the multi's. They didn't pay for the hours it took to record multi's, they purely payed for a mixed/mastered file. Now that I charge by the hour, I look at it more along the lines of they payed for every hour they spent tracking those multi's, excluding the drum replacement, and as Jordon says, just put the original kit back in, but don't give them the sampled/replaced drums because they didn't pay for the samples.