Ciao,

I'm afraid the resource leveling command will not do what you seek. If you assign Giovanni at 100% to task B - Project will flag the resource as overallocated but it will simply delay Task B until the finish of Task A. If you know that Giovanni is already committed at 25%, can you just assign Giovanni at 75%, enter the 24 hours of work and Project will calculate 4 days duration?
Try splitting the screen to show the Task Form in the lower pane and assign Giovanni at 75% and enter 24 hours of work on assignment.

I hope this helps.