I don't know anything about share option schemes really but thinking out loud, did you own the shares outright or were they still being held in a fund by your company? If you did get the share allocation at a preferential rate in the first place that was dependant upon you holding them for a fixed period (eg 5 years), you may have lost that discount by cashing them in early. i.e. you have now forfeited that discount and effectively had to pay the market rate prevailing at the time that they were allocated to you. So your profit of $20 per share is reduced because you hadn't qualified for the initial discount by cashing them in early.
Other than that, I can't come up with any reason for the missing £500.
How's your cold today, Jools? Have you got a headache now?
The only think I could think it might be is if the broker account requires a minimum balance to be held (eg $1,000 which would correspond to about £500).
Either way, a call to the broker should sort it out.
Jools, I did exactly what you did a few years ago (US company an' all) and I got the full proceeds (i.e. profit), actually I didn't even get tax deducted, I had to account for that to the Revenue later.
Thanks for your help everybody, just to clarify a few things that people have asked.
The options are all fully vested now (25% per year over 4 years) so I 'own' them all without any time restriction. The price that I sold them at was confirmed as $37.994 (see, I told you it was as near to $38 as to make no difference). The schedule of brokers charges is very simple and their fees are nowhere near £500 - in fact less than $100.
I have two types of stock option 'approved' and 'unapproved'. The approved ones mean that I have already paid tax on them and I am not liable for any further tax, the unapproved ones mean that I pay 40% tax on the difference between the grant price and the sale price. These were a block of unapproved ones, hence the 40% tax.
The bank swear that no negotiation fee is applied (I've been through that before when someone sent me a dollar cheque).
It all confirms to me that someone, somewhere has cocked up.
I've got a cold at the moment, and unwisely went training last night, so I was too cream crackered to call the broker, but I will now I've got a bit more knowledge.