Am I entitled to cancel the sale of a plot of land to my brother after not receiving the full agreed-upon amount for its purchase, especially after land prices have risen significantly, and what should I do in this situation?

The original ruling is that a sale is binding and enforceable once the two parties have separated, and neither party may unilaterally annul it except with a legitimate Sharia-compliant reason. Therefore, the sale is binding on you, and you are only entitled to the remaining balance of the original agreed-upon price of the plot, even if the price has changed. Your brother's procrastination is a sin, but it does not justify your annulling the sale or taking an increase. It is advisable to negotiate amicably with your brother, and perhaps he will willingly pay you an increase, while being keen on maintaining family ties. The fact that the land has not been registered in your brother's name does not affect the validity or enforceability of the sale.