Anyone use interactive brokers?
Yes, for quite a while, and that's not really how it works. Unless you see $2.84 bid, you're not guaranteed a fill selling at $2.83.
If you're selling at $2.83, and the bid is $2.83, you may sell everything, you may sell some, and you may sell none, depending on how many competing shares were also offered at $2.83. These examples are not unique to IB, just how the market works.
IB doesn't modify your orders. However multiple buyers may have purchased some shares of your order up until the entire order is filled. Sometimes you may get a fill price better than your order, but you'll never get it worse than your order. Last I remember, for a single order (except stocks <$1 or $2), the base rate is $1 and it's $.005 per share after that. If you modify your order, then that makes it a new order and you'll have another $1 minimum base charge. Also, if the order isn't filled by the end of the day, you may have the $1 base charge at the beginning of the next day again (not sure.)
It sounds like your order was on the edge of the market range, and like it or not, the professionals trade with themselves (and they have incentives to do so) before the small independents get a piece of the pie. (one more thing, if your order was "hidden/iceburg", that makes filling it less efficient and can take longer, possibly missing the opportunity to execute the entire order at the time.)