What is the ruling on the annual bonus offered for maintaining a specific cash amount, on the condition that it does not fall below this amount to qualify for the bonus?

There is no objection to accepting this bonus, as it is a conditional gift intended to encourage the preservation of money. It is permissible to give and take it, and there is no difference between a lump sum and a percentage, so long as it is known. The gift here is known because it is a percentage of a known amount, and a gift conditioned on a permissible act is valid. This is the choice of Shaykh al-Islam Ibn Taymiyyah and Ibn al-Qayyim. It must be noted that if the saved money reaches the nisab and a full year passes over it, zakat becomes obligatory on it.