ORA-01722: invalid number, how to define a numeric variable

This is running in SQL*Plus script.  (I will greatly appreciate any help, but please don't provide guesses if answering)

I intend to pass parameter &1 through a concurrent job where parameter &1 will be assigned to variable p_Inv_Org.

p_Inv_Org is the variable that is used in the sql statement.

ORA-01722: invalid number error is given for the following snippet of code.

Snippet of SQL*Plus code looks like:

-- Passed Parameters Management.

  UnDefine p_Inv_Org;

Define p_Inv_Org = '&1';

.

.

.

Select from... where msi.Organization_Id = 'p_Inv_Org'

So, question is: how do I get the sql statement, using SQL*Plus script to execute?