I am not an accountant and think you should consult one about your case. I believe that you can only defer tax on share options for 7 years. From what I know, the tax on exercise is fixed (at the date of exercise and at the exchange rate on the day). If the exchange rate is worse now, then when you sell the shares, you can claim a capital loss for the difference in the total value of the shares at the date of exercise (incl then-exchange rate) and the total value when you sell (incl current exchange rate).
From your numbers, if you sold all the shares today at 7kEuro, then you will have paid 8.8kEuro in income tax (or 7k, it's not clear from your post) and you will have capital loss to carry forward of 13k Euro (20-7). That doesn't take into account the actual exercise price, which you would have deducted prior to calculating income tax.
The payment of the income tax essentially means that your CGT base becomes the market value at the date of exercise, notwithstanding you didn't actually pay that much (you paid the exercise price + income tax).
I have to stress that I am not an accountant and this is amateur advice only. You should consult a professional.