Article 3 of labor code of the Philippines?

Before,our basic salary is given to us 100%.

Revision has been made recently as:

Basic Salary = amended amount plus representation.

Representation must be forwarded to HR. This requires official receipt indicating the Tax Identification Number of the establishment; official receipt must be issued under the name of our company. If one cannot liquidate official receipt as his representation,then HR will just pay the amended amount which is lower compared to our basic salary as stated on our contract. Revision has been made by HR for purposes of trimming down tax dues.

My question is:

Is it lawful to amend the specified basic salary on our contract?