Is it permissible to sell devices with varying profit margins among customers for the same product, while the market price is stable, and is it obligatory to inform the customer of the device's purchase cost when asked about it?

Your question encompassed four matters:

1. The permissible profit margin according to Sharia.

2. Selling the same item at different prices to different individuals is permissible, with no objection.

3. Increasing the price written on the invoice beyond the actual price is forbidden, as it constitutes lying and deception.

4. Informing the buyer of the cost price is not obligatory; what matters is not to lie about the stated price.