Is it permissible (halal) or forbidden (haram) to program an operating system specifically for beauty salons and massage parlors (SPAs) and lease it annually to these centers, while distinguishing between women's salons (where male employees are not allowed) and men's salons (where female employees are not allowed)?

The basic principle is that it is permissible to work on programs that can be used for both permissible (halal) and impermissible (haram) purposes, unless it is known or highly probable that their use will predominantly be for haram. If the predominant benefit derived from the sold item is an impermissible benefit, then it is not permissible to sell it. If a salon adds an impermissible service or uses the program in an impermissible manner, the sin falls upon the one who does so, not upon the worker, as Allah Almighty says: "And every soul earns not [anything] except for itself, and no bearer of burdens will bear the burden of another."