You could ensure that each number is really unique by looking for it back in the output file...
test=`cat amount.log`
touch random_numbers.log
for i in $test
do
while true
do
num=$((RANDOM%10000+20000))
grep $num random_numbers.log > /dev/null
if [[ $? -gt 0 ]]
then
break
fi
done
echo $i $num >> random_numbers.log
done