Opponent changes rating during game, do my potential win-points change also?

I would like to know whether the ratings you will win/lose when you enter into an 'Online Game' are locked in as soon as you accept/create a challenge, or whether the points you will win/lose are variable depending on the ratings that you both are at the end of said 'online game'... here is a scenario to explain what I mean:

Say im 1600 and I decide to play a 1800. (I like to play higher rated players than myself to improve, and gain more points)

Lets say he/she has many other games on the go, and as the game draws to a close:

My rating is still 1600, but he/she has had many awful games whilst we were playing and his/her rating in now 1400.

I then check-mate with my superb skill. Now do I win the points equivalent to beating an 1800 rated player (which was the original terms to which I agreed to enter into the match), or do I win the points equivalent to beating a 1400 player?

Please provide a detailed response as to how this is calculated as I am very interested.

Much Appreciated.

It's based on the scores you have when the match is over. It's as simple as that.

Easy money! thanks