budgeted hours / budgeted hours/day conflict

If an activity is running for 20 days at 8 hours per day, but one of the resources is going to be involved for just 10 days at 8 hours per day, how can that be reflected in the budgeted resource hours? If I set the budgeted hours to 80, it shows the resource working only 4 hours per day. If I set the budgeted units per time at 8 hours per day, it shows 160 budgeted hours.

Dave