Your code doesn't store the nick who received the money anywhere. If you are only storing the total sent to everyone, just use a variable:
on *:text:[CASH SEND]*:#echo: {
if ($7) { inc %cashsent $remove($5,$chr(36)) }
}
If you want to store the money received for each nick, you'd be better off using hash tables, imo. Still...
on *:text:[CASH SEND]*:#echo: {
var %result = $read(txt\ingame_stats\sentamount.txt,ns,$7)
.write -l $readn txt\ingame_stats\sentamount.txt $7 $calc(%result + $remove($6,$chr(36)))
}