Looks like you are trying to set the variable vCurrency to the minimum cd_nis value in script. However, this expression will not be evaluated in the script. It will only be evaluated in the front end when the script is finished.
You need to Peek() the value from the table, like this:
SQL SELECT ....
LOAD MinString('cd_nis') as Min_NIS
Let vCurrency = Peek('Min_NIS');
DROP Table Min_NIS;
Now use vCurrency as before.
Logic will get you from a to b. Imagination will take you everywhere. - A Einstein