Shell Programming and Scripting

I have a set of user name password pairs in one file and I have a set of files which have user names and a random string as the password. How can I read the user name and password from the first file and replace the random password in the second file.

my list file looks like this

user1, pass1
user2, pass2
user3, pass3

the other files (the ones I want to replace) look like this.

line 1 bla bla bla
line 2 userx
line 3 skip line
line 4 skip line
line 5 randompass
more lines...

so far I have this awk command that does replaces one password at a time

gawk /"user1"/{l=NR}l&&NR==l+3{sub(/"randompass"/,"\"pass1\"")}1 file1.txt > newfile1.txt

The problem is that not all the files contain user1 so I need to grab the first row of my list file then replace in all the files that have user1 then read the second row replace and so on.
All ideas are welcome. Thanks.

Code:

open FH,"<b.sh";
while(<FH>){
	@arr=split(",",$_);
	$hash{$arr[0]}=$arr[1];
}
close FH;
open FH,"<a.pl";
while(<FH>){
	$pass=$hash{$1} if (m/(user.)/);
	if(m/pass/){
		$_=~s/randompass/$pass/;
		print $_;
	}
	else{
		print $_;
	}
}

Quick question. Does this assume that I know the old password (random password)? If that is the case the code will not work in my case. The only thing I know about the previous password is that it is two lines after theuser name so that is why my awk code above adds 3 to the NR. So I'm looking for a way to pass the read user name into my awk.
Thanks