engineers are rarely businessmen and thus try to use engineering practices for business purposes i. e. "time bombs". This, in my opinion, is wrong.

a situation

Prahlad put time traps into his system and the following happens

customer: "the darn thing does not work"

Prahlad: "pay me and I will make it work"

customer: "I will not pay for something that does not work"

Prahlad: "then we are stuck"

result customer has no unit and Prahlad has no money.
Possible result: customer sues to get down payment back.

Given result everybody is unhappy.

even if the customer pays up and Prahlad reenables the unit the customer will be unhappy

I would recommend strongly against "time bombs" UNLESS their presence is stated clearly in a contract signed by the customer before you have placed the first component on the schematic or written the first line of code. Terry's suggestion might work.

Erik