Is it permissible to take an increase in the price of charging mobile message credit without informing the employers, in exchange for designing a computer program for them for which they did not calculate the true price?

It is not permissible to take an increase without permission; rather, one must fulfill the trust and not betray it, as Allah Almighty says: "Indeed, Allah commands you to render trusts to whom they are due," and the Prophet, peace and blessings be upon him, said: "Render the trust to the one who entrusted you, and do not betray the one who betrayed you." Betraying a trust is one of the signs of hypocrisy. A mere claim of right is not sufficient; rather, "the burden of proof is on the claimant, and the oath is on the one who denies."