10% - just to hold an apartment!

That's like asking for problems. No check without proper paperwork.

Don't do it!!!

I agree, don't do it. Something don't jive.
It is possible the seller's been through 2 blown in contracts and his lawyer's barking for some early cash. Still, that's not your problem.
You know, the seller has to pay his attorney to come up with a contract regardless. If you buy it, someone else buys it, it is drawn up with minor changes depending on what you negotiate easily changeable or paragraph inserted here or there.

The only possible scenario I could envision if your heed over heels in love with this apartment and want to make it happen so bad is to keep your deposit in escrow with YOUR attorney. But that's even opening a can of worms. And then of course your gonna have draw up some document on conditions of escrow. But maybe just suggesting it puts seller at ease as to your intentions to close.

Still, it is too much a buyer's market to make such a seller demand.

DO NOT DO THAT. It sounds like you are being scammed, no joke.

if your atty has not told you emphatically NOT to do this, get a new atty

In NYC this is not done. I believe that in some other states there is something like this, but the entire process is different in those states. Our RE standard operating procedures are not set up for what you have been asked to do. The more you do out of the ordinary in a RE transaction the more you ask for trouble--serious trouble since the dollar amounts are high. Do not do this. Period. Any lawyer should tell you this. The purposes of such a payment are unclear and the terms under which a seller could keep your money are unclear. If there are no situations under which they could keep the money, then the check should not be written to begin with. None of this makes sense in a NYC RE transaction for residential property.

I have been involved in this process several times, but it never exceeded $1,000 and it never occurred in NYC. I just can't believe any REBNY broker would even want to participate in a deal like this. Go visit your broker, tell her to fuck off, slap her on the ass (she likes that), then move on.