It looks like there are several errors
First you calculate a variable named round2
round2 = interest_amount2.toFixed(2);
and then you display the value of round:
label_interest.text += String(round) + "r";
Second, total_amount is the sum of non rounded values, and is not rounded itself:
total_amount = interest_amount2 + a_amount;
Maybe you could calculate every numeric values, then display rounded values, like in this example:
label_amount.text += a_amount.to_fixed() + "\r";
Note you don't need to cast Numbers to Strings; it is done implicitly when you concatenate them with a String.