You do 80% of the functionality with 20% of the required code and pretend that this is what the project manager asked for. Likely they'll never notice the missing bits which may have seemed like a good idea at the time .If they insist you do 80% of what remains with another 20% of the code.

But with some luck your project managers' butterfly mind will have flitted on to greener pastures and everybody is happy. No need for Grug to reach for his club (while remaining calm).