Our Individual Savings Accounts (ISAs) are not flexible, which means that a withdrawal reduces the annual allowance permanently.
Consider the following example:
In October 2019, you've already contributed £17,000 to your Exo ISA. As the annual ISA allowance is currently £20,000, you have £3,000 of contributions left. In November 2019, you need to withdraw £5,000 to cover an unexpected expense.
The balance of your ISA portfolio is now £12,000 and you might think that you're again able to transfer £8,000 into this portfolio until the end of the 2019/2020 tax year.
However, as Exo ISAs are not flexible, the allowance is what is was before you made the withdrawal: £20,000 - £17,000 = £3,000.
Therefore, the maximum amount you'll be able to transfer into this ISA in the current tax period is still £3,000. This means that the maximum invested amount for this tax year will finally be £12,000 + £3,000 = £15,000.
Please note that this doesn't prevent you from investing more money into a General Investment Account (GIA).