as far as i know,
Hitbtc will allow you do the trade for more than 10 Coin.
Bittrex will allow you do the trade if your total sells exceed 50.000 satoshis.
they all have restrictions in the same range. usually it is amount of bitcoin you get in return and in 90% of exchanges it is smaller or equal to 10,000 satoshi.
for example in case of bittrex it is 50k as you said. but with that said, OP's question doesn't make any sense at all!
if a coin is worth $300 that means what he is calling "leftover" and says it is 0.5 token is worth $150 or ~0.034
BTC and that is way beyond that limit!
actually i understand what he said,
i got same issue just like him,but his question is questionable indeed.
he said what will happen with my 'leftover' (0,9 token for example) and if the price for each token is $300 .
that is a lot in my opinion and absolutely many people will complain about that.
but the most important thing that we must know,what is our current price ?
for example my own experience.
i have 7002 TKR token and i sold it on hitbtc.com,
in there i only can sell 7000 tokens,and i can not sell the rest of it(4 tokens).
so,what if 1 Token price is $300 ? how much it will be ?
the real problem in here is,that will not happen and the reason they do not allow it because it's too little.
because current price is around 0.0004 ETH.
so,do not think about it too much.
and read about their service rules and terms before doing anything.