Is it permissible to inflate the amount received in exchange for an engineer's seal on blueprints only, without performing any other engineering work?

The money paid to obtain a stamp or signature is not for the stamp itself, but for its implication, which is the engineering office's certification of the accuracy and soundness of the blueprints.

The stamp must be placed on something that is correct and sound according to regulations; otherwise, certifying it would be considered a false testimony, deception, and fraud, as Allah Almighty says: "Their testimony will be recorded, and they will be questioned" (Az-Zukhruf: 19).

If the transaction is free from the aforementioned caveats, then there is no objection to the engineering office taking a sum for reviewing the blueprints and ensuring their compliance with standards. This amount would be according to the agreement between the two parties.