How can I do assign a Hex value to variable from other variable.

I mean I have this:
buffer is a char array
aux is a chara array

buffer[i] = atoi(aux);

atoi(aux) return for example 40, then I want to see the value 40 on the buffer[i], but is not the case the value is 28 (is 40 in hexadecimal)

How can I get the value stored be 40????!!!!!!!

I know if I do this value = 0x40, then 40 is stored but how can I do getting the value from other variable.

I really need this, thanks a lot for your help!!!