Imprest given to directors in cash in a single day more than ₹ 5,00,000.

Better to read sec 269st with 269ss...

According to me both sec 269ST and 269SS will not apply on company because both sections define the mode of acceptance of money not payment.
Sec. 40A(3) also will also not apply because Imprest given is not business expenditure.

In My opinion the co. can pay imprest in cash to directors.

But one thing has to keep in mind, if it is a pvt. co. and director to whom Imprest was given holds more than 10% of equity share then two things has to be checked
1. there should be business expediency for giving imprest and
2. the amount is utilized for business expenditure
if above condition are not satisfied the imprest given may be treated by AO as deemed dividend to director by the virtue of sec 2(22)(e).