Perl, 50 bytes

$_=<>;s|\.||;$_<<=1;m|\d{3}$|;printf'%.2f',"$`.$&"

The input is expected at STDIN. It must contain the decimal separator with two decimal digits. The output is written to STDOUT.

Examples:

• Input: 20.96, output: 4.19
• Input: 12.34, output: 2.47

Ungolfed version:

$_=<>;
s|\.||; # multiply by 100
$_ <<= 1; # multiply by 2
m|\d{3}$|; # divide by 1000
printf '%.2f'," $`.$&"

First the number is read from STDIN. Then the decimal dot is removed, that is multiplied with 100. Then the amount is doubled by a shifting operator. Then the decimal dot is reinserted and the result is printed and rounded to two decimal digits.

Perl, 50 60 bytes

$_=<>;s|\.||;$_<<=1;$_="00$_";m|\d{3}$|;printf'%.2f',"$`.$&"

The input is expected at STDIN. It must contain the decimal separator with two decimal digits. The output is written to STDOUT.

Update: The step $_<<=1 removes leading zeroes. Therefore m|\d{3}$| would not match for bills < 1. Therefore ten bytes $_="00$ were added, Now even 0.00 works.

Examples:

• Input: 20.96, output: 4.19
• Input: 12.34, output: 2.47

Ungolfed version:

$_=<>;
s|\.||; # multiply by 100
$_ <<= 1; # multiply by 2
$_="00$_";
m|\d{3}$|; # divide by 1000
printf '%.2f'," $`.$&"

First the number is read from STDIN. Then the decimal dot is removed, that is multiplied with 100. Then the amount is doubled by a shifting operator. Then the decimal dot is reinserted and the result is printed and rounded to two decimal digits.

50 bytes, if bill ≥ 1:

If x.xx is greater or equal than 1.00, then 10 bytes can be removed:

$_=<>;s|\.||;$_<<=1;m|\d{3}$|;printf'%.2f',"$`.$&"