Is the 5,0 dinars that the partner borrowed and was unable to repay, and proposed that it be part of my share in the bus damaged by the loss, considered part of the share and thus bears a portion of the loss, or does it remain a separate loan?

The intent of the question is not entirely clear. However, if it is agreed that the debt becomes a share in the partnership, then this is valid as a sale from the brother of a share of the bus in exchange for his debt. Accordingly, the questioner is entitled to the profit from this new share from the date of agreement and bears its loss if it occurs, as profit is in exchange for bearing loss and guarantee.