Quote:
How much was he asking for this one originally?
About $9, as it happens (I'm rounding the amounts a bit - it was really 500 rubles), so I guess I might just make up the difference to that

Originally, there was a line of 5 coins, clearly all of similar origin, priced for 500 rubles (~$9) each.
I bought the pentagonal 1/8 riyal because I wanted a pentagonal coin and wanted a Yemenite states coin and this was both, and then next week I bought the huge 1/2 riyal because it was large and silver (but it turned out to also have an obvious removed mount, which I didn't immediately notice when I bought it).
The other three stayed there for the next five weeks or so, while I occasionally visited the dealer for more picking from his excellent 30 ruble (~$0.5) bargain bin.
On Friday, I finally received the money I was supposed to earn way back in November.
Just as I planned, on Sunday, I came to a nearby store for a Liechtenstein coin I wanted, but upon arrival I unexpectedly found out that the coin had since been sold.
So I suddenly ended up with $40-ish of extra spending money.
Anyway, I go to the place with the bargain bin, get a few coins from the bin (it had mostly been picked out, so there's not much still left) and a few cool commemorative crowns (mostly from random minor British territories).
The dealer still wants to sell me the Yemenite coins, because nobody else appears to be any interested in them, so he says that I can have
two for 500 rubles (this comes out to something like $4.4 per coin, which is what I rounded down to $4 in the OP).
Now, due to a misinterpretation of the catalog (and because it looked so different from the worn silvers nearby), I was under the impression that this particular silver coin was aluminium (as I mentioned in the OP) since pretty much the first time I saw that bunch.
So I said as much to the dealer, and he said that he could give it to me for an extra 100 rubles (a bit under $2).
Anyway, I bought the coin, came home, tried to attribute it, and finally realized that it was, in fact, silver and just that good.
At which point I started the thread (because I really had no idea what to do).
...Sorry for such a large explanation.
TL/DR - I guess I should just give him the extra 400 rubles and explain that the coin was, in fact, silver. Will try to do just this on Saturday (his shop won't be open until then).