Is the guarantee of the land plot's value incumbent upon the questioner or upon his brother who exploited the land owner's delay in registering it in his name and sold it?

The responsibility for your brother's encroachment on another's land falls upon him, and you are not his guarantor. Your colleague was negligent in completing the administrative procedures to transfer the land into his name. He was aware of your brother's travel and even assisted him. Therefore, you have no connection to the matter, and no responsibility falls upon you, as Allah Almighty says: "And no bearer of burdens will bear the burden of another."