You have not specified which user variable you are using. User has strings, reals and integers ...
such as
USERS1
USERR1
USERI1
For this example I will simply use x in place of the data type. For diesel it makes no dofference anyway.
$M=$(getvar, userx1)
You need to insert this into the button command, where the scale factor is requested, simply use that bit of code. For example, when you insert with a scale of 50 ...
-insert dwgname;\50;50;\
use this instead..
-insert dwgname;\$M=$(getvar, userx1);$M=$(getvar, userx1);\
Good luck