Ayman,
Your client pays you, you owe loyalty to him as long as it is not agaiinst the law. I agree with Dieter with regard to if the relation with the client is not open this is calling for problems.
On the other hand I can understand your client and you not wanting to disclose all information to others. The way you present the issue I am assuming you represent the party responsible to creating the schedule, the party in control of the means and methods.
If you represent the General contractor in case of the traditional contract arrangement or the Project Manager in case of a Project Management Agreement then you might want to hide some information to others. For example your cost information and even available float.
As the party who bears the most risk you do not want others to take away float because it is "free", that is a lie all Contractors know, use of float comes at a cost. It is a lie of American creation but not accepted in all countries. What if the owner and or subcontractors consume all float because it does not theoretically delays the job? You got to make sure this confidential information is not leaked and miss-used, make sure there is no way others can get to it.
If your software is so deficient with regard to keeping confidential information you shall consider keeping two version of the schedule. If you still need to hide data such as Float and you cannot hide it as you should then use PDF reports or use other software to transfer data where you have better control of what the other user can or cannot see/edit in any table, any view. I allow Subcontractors to see everything and edit reports except I deny access to any float value, all they can see is a shaded cell/value. I use other software but that you cannot control such basic need would be a serious limitation.
Best regards,
Rafael