fn_varbintohexstr seems to return different values when used in a stored procedure

I created a stored procedure to generate a SHA1 password hash:

alter procedure GetPasswordSp (
   @pUsername nvarchar(50), 
   @pPasswordClear nvarchar(15), 
   @pPassword char(50) output
   )
 AS
   
if @pusername is null
   RETURN
   
set @pPassword = SUBSTRING(master.dbo.fn_varbintohexstr(HASHBYTES('SHA1', ltrim(rtrim(@pPasswordClear)))),3,50) 

RETURN

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Its simple enough... when I build a test it runs fine and the value that is returned is what I expect... when I run it as a stored procedure I get a different return.

I assumed that there was an extra character begin padded in my code, so I changed it from  a hash to simply display the hex results so I could see the extra chars

I test manually again:  username=username, pw = Buckwheat

RETURN VALUE: 0x4275636b7768656174

Again, this is what I expected...

I copy/paste the same code into the SP

master.dbo.fn_varbintohexstr(CAST(ltrim(rtrim('@pPasswordClear')) AS varbinary))

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and pass the same values

RETURN VALUE: 0x4200750063006b0077006800650061007400          

NOTICE THAT IF YOU TAKE OUT THE zero-zeros THE VALUE IS THE SAME....  

Any suggestions as to how to fix?

The hash key (SHA1) is going to be generated in SQL and also on a web page to validate passwords, if they don't match, well, you can imagine the problems.