How the total price of Market Buy order is determined
When you confirm Market Buy order the system takes current ask+10% of funds to ensure order execution and to protect it in thin market. System holds them until the order is executed and the leftover is returned to your portfolio.
Why there is a leftover
Market order differs from Limit order because you are buying assets at market value. But current ask in a thin market is constantly changing and it will differ from order fill to order fill. If you are buying a 1000 COV the order is filled by smaller orders, so out of this 1000 COV you buy 275 COV for 0.00014669 BTC, then 68 COV for 0.00014670 BTC, then 200 for 0.00014677 BTC and it goes like this until the order is filled. This leads to an increase in the total price of your order but, because 10% were added to ensure the execution of the order, this increase won't hinder the execution of your order. And if this increase didn't go above current ask+10% you will see a leftover after the execution.
~0.002640 BTC were withheld by the system to execute this Market Buy order but after the execution 0.0002398 remained so they were returned to the portfolio.
What happens to your balance if the order goes above the current ask+10%
If the total price of the order goes above current ask+10% that the system was holding, the order will be executed but the balance of your portfolio for this asset will be negative and withdrawal function will become unavailable until you transfer more funds to restore balance for this asset.