I want to write a letter to an institution I interviewed, telling them that:

I would very much like to be considered for that vacancy. But I have to make a quite decision because I have got an offer from another place who wanted me to reply immediately. I am in a dilemma because I don't want to lose the opportunity to work for you, and I can't afford losing an offer by keeping them waiting so long.

How shall I put it clearly and appropriately? I don't want to sound like I am using leverage to get that job...

Please help me...Thanks a lot!!