I think a bit of clarification is in order:
You are contracted to work to 76 flying hrs pcm (depending how long the month is ie 31 day month is 78hrs).
Any hours you do over this are regarded as over time.
You may well be (and often will be) ROSTERED to fly into over time for which you will be paid 315dhs (85$) per hour for F/O and 460dhs (125$) for Capt (I think?).
However, once you have your roster, you are not OBLIGED to accept any more duty/flying, although you can be phoned on a day off and asked (as is the norm in most companies). If you are not hitting overtime or cant be bothered, just say NO, if you want to do it because you're a bit skint that month, say YES please, and make sure you also get your day off payment!
Nothing sinister about it all, but put simply you have no choice in accepting overtime in your allocated roster, but you do once the roster has been issued!
Hope this helps?