if you will travel to SG as tourist, your country immigration will ask for a return ticket. you should have 1 even if it's dummy.
when in SG, no problem @ all just present your ticket and visa on your scheduled trip to qatar.
but before doing so, please check first the company you'll work with in the internet or from friends you know here or uR family to be sure that the company is good enough for you.
just a tip... Good luck!