Pop up message

Question

 Rent  textbox                    1000

Tax    textbox                    123.6

Gross rent textbox               1123.6     

Tds     textbox                     100

Net rent  textbox               1023.6


suppose the net rent is rs 1023.6 if suppose client pays the rent rs 800 only remaining 223.6 has to pay for the month.

so i have to show the pop up message you have to pay remaining amount of rs 223.6 for this month.  how to do.please help me. using csharp.

thanks,

Regards,

Narasiman P