Every proposal that is based on a fixed USD amount, from the moment it is presented, contains immediate market risk...

If you require USD 1000 and current Dash price is $4.00, then you ask for 250 Dash. You, the proposal owner, take on immediate market risk - when your proposal gets paid, those 250 Dash will be worth either more than or less than the required $1000.

Or you can transfer that market risk by convincing a service provider to accept Dash rather than fiat, in which case the service provider will end up with more than or less than the $1000.

The budget system itself does not, and can not take on market risk - it pays 250 Dash regardless of market price. Rather, the market risk must be borne by either the proposal owner or the service provider, and they will do better or worse depending on the market.

With multiple payments further in the future, the market risk is of course magnified.

Edit: whoever assumes the market risk should be entitled to any upside reward, just as he is subject to any downside loss.