All salaries in Qatar are paid in Qatar Riyals which are tied to the US dollar.
Fluctuations do occur and can sometimes be large. I recently worked in Abu Dhabi for 3 months and had agreed a rate that equated to £1,000 per week (+ accommodation etc) in AE Dirhams. The exchange rate when I agreed the figure was AED6.8 to the £. By the end of the contract, the rate was AED 7.219 to £. A change over 6% in 3 months.