Nothing is guaranteed in the financial markets due to the varying levels of price volatility on a daily basis. All positions are filled on the basis of market execution.
Market execution means all trades will be executed at the best available price in the market at the time you take or close your order. This ensures that you will always be filled at the best price which might not always be the price you requested.
At times, you will not be able to execute at your desired price, either because the price has changed before the trade was executed or the size of your trade exceeds the volume available in the market at that price currently.