It is not possible to get A Dollar from this item.
Using this item in a boss room to spawn a fly after defeating the boss will enable the player to leave the room and come back, facing another boss battle and obtaining another item (This seems to have been fixed on The Binding of Isaac Rebirth, or at least, it didn't work on the Vita version).
The type of heart dropped is affected by the current room (like a normal Slot Machine), e.g. an eternal heart if in the Cathedral Room.
Using the Portable Slot just before exiting a room will negate its effect.