If it's a special order, in-demand shoe I personally don't see any reason not to expect to pay 100% up front. 50% is more than fair - probably even generous.
Now, if they don't come as promised (rarely, I hope) - well, then depending where/how the error was made, that needs to be worked out between the manufacturer and the vendor and the appropriate party needs to stand up for the oversight.
I love my Cigar Brixtons enough to not be too sad about the Rydens. Cheers Mike.