From the Super Mario Wiki
A Halfway Dice Block is an item that is purchasable in the Item Shop of Mario Party DS. As its name suggests, this item has a maximum dice roll of 5, half of numbers of a normal Dice Block, which goes up to 10. Also, in Kamek's Library, when the player lands on a ? Space at the very top of the board, Kamek would appear and replace the player's items with three Halfway Dice Blocks. It only costs one coin and it is the only item which is cheap.
The Halfway Dice Block has the same effect as the block used when characters in other Mario Party games are subject to a Cursed Mushroom.